| Message ID | 20230305205628.27385-2-ubizjak@gmail.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | locking: Introduce local{,64}_try_cmpxchg | expand |
On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. Can you give an example of where we are passing an incompatible pointer? That sounds indicative of a bug in the caller, but maybe I'm missing some reason this is necessary due to some indirection. > Fixes: 29f006fdefe6 ("asm-generic/atomic: Add try_cmpxchg() fallbacks") I'm not sure that this needs a fixes tag. Does anything go wrong today, or only later in this series? Thanks, Mark. > Cc: Will Deacon <will@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Boqun Feng <boqun.feng@gmail.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > --- > include/linux/atomic/atomic-arch-fallback.h | 18 +++++++++--------- > scripts/atomic/gen-atomic-fallback.sh | 2 +- > 2 files changed, 10 insertions(+), 10 deletions(-) > > diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h > index 77bc5522e61c..19debd501ee7 100644 > --- a/include/linux/atomic/atomic-arch-fallback.h > +++ b/include/linux/atomic/atomic-arch-fallback.h > @@ -87,7 +87,7 @@ > #ifndef arch_try_cmpxchg > #define arch_try_cmpxchg(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -98,7 +98,7 @@ > #ifndef arch_try_cmpxchg_acquire > #define arch_try_cmpxchg_acquire(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg_acquire((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -109,7 +109,7 @@ > #ifndef arch_try_cmpxchg_release > #define arch_try_cmpxchg_release(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg_release((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -120,7 +120,7 @@ > #ifndef arch_try_cmpxchg_relaxed > #define arch_try_cmpxchg_relaxed(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg_relaxed((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -157,7 +157,7 @@ > #ifndef arch_try_cmpxchg64 > #define arch_try_cmpxchg64(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg64((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -168,7 +168,7 @@ > #ifndef arch_try_cmpxchg64_acquire > #define arch_try_cmpxchg64_acquire(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg64_acquire((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -179,7 +179,7 @@ > #ifndef arch_try_cmpxchg64_release > #define arch_try_cmpxchg64_release(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg64_release((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -190,7 +190,7 @@ > #ifndef arch_try_cmpxchg64_relaxed > #define arch_try_cmpxchg64_relaxed(_ptr, _oldp, _new) \ > ({ \ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ > ___r = arch_cmpxchg64_relaxed((_ptr), ___o, (_new)); \ > if (unlikely(___r != ___o)) \ > *___op = ___r; \ > @@ -2456,4 +2456,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v) > #endif > > #endif /* _LINUX_ATOMIC_FALLBACK_H */ > -// b5e87bdd5ede61470c29f7a7e4de781af3770f09 > +// 1b4d4c82ae653389cd1538d5b07170267d9b3837 > diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh > index 3a07695e3c89..39f447161108 100755 > --- a/scripts/atomic/gen-atomic-fallback.sh > +++ b/scripts/atomic/gen-atomic-fallback.sh > @@ -171,7 +171,7 @@ cat <<EOF > #ifndef arch_try_${cmpxchg}${order} > #define arch_try_${cmpxchg}${order}(_ptr, _oldp, _new) \\ > ({ \\ > - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \\ > + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \\ > ___r = arch_${cmpxchg}${order}((_ptr), ___o, (_new)); \\ > if (unlikely(___r != ___o)) \\ > *___op = ___r; \\ > -- > 2.39.2 >
On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > Can you give an example of where we are passing an incompatible pointer? An example is patch 10/10 from the series, which will fail without this fix when fallback code is used. We have: - } while (local_cmpxchg(&rb->head, offset, head) != offset); + } while (!local_try_cmpxchg(&rb->head, &offset, head)); where rb->head is defined as: typedef struct { atomic_long_t a; } local_t; while offset is defined as 'unsigned long'. The assignment in existing try_cmpxchg template: typeof(*(_ptr)) *___op = (_oldp) will trigger an initialization from an incompatible pointer type error. Please note that x86 avoids this issue by a cast in its target-dependent definition: #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \ ({ \ bool success; \ __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \ __typeof__(*(_ptr)) __old = *_old; \ __typeof__(*(_ptr)) __new = (_new); \ so, the warning/error will trigger only in the fallback code. > That sounds indicative of a bug in the caller, but maybe I'm missing some > reason this is necessary due to some indirection. > > > Fixes: 29f006fdefe6 ("asm-generic/atomic: Add try_cmpxchg() fallbacks") > > I'm not sure that this needs a fixes tag. Does anything go wrong today, or only > later in this series? The patch at [1] triggered a build error in posix_acl.c/__get.acl due to the same problem. The compilation for x86 target was OK, because x86 defines target-specific arch_try_cmpxchg, but the compilation broke for targets that revert to generic support. Please note that this specific problem was recently fixed in a different way [2], but the issue with the fallback remains. [1] https://lore.kernel.org/lkml/20220714173819.13312-1-ubizjak@gmail.com/ [2] https://lore.kernel.org/lkml/20221201160103.76012-1-ubizjak@gmail.com/ Uros.
On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > Can you give an example of where we are passing an incompatible pointer? > > An example is patch 10/10 from the series, which will fail without > this fix when fallback code is used. We have: > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > where rb->head is defined as: > > typedef struct { > atomic_long_t a; > } local_t; > > while offset is defined as 'unsigned long'. Ok, but that's because we're doing the wrong thing to start with. Since local_t is defined in terms of atomic_long_t, we should define the generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still have a mismatch between 'long *' and 'unsigned long *', but then we can fix that in the callsite: while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) ... which then won't silently mask issues elsewhere, and will be consistent with all the other atomic APIs. Thanks, Mark. > > The assignment in existing try_cmpxchg template: > > typeof(*(_ptr)) *___op = (_oldp) > > will trigger an initialization from an incompatible pointer type error. > > Please note that x86 avoids this issue by a cast in its > target-dependent definition: > > #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \ > ({ \ > bool success; \ > __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \ > __typeof__(*(_ptr)) __old = *_old; \ > __typeof__(*(_ptr)) __new = (_new); \ > > so, the warning/error will trigger only in the fallback code. > > > That sounds indicative of a bug in the caller, but maybe I'm missing some > > reason this is necessary due to some indirection. > > > > > Fixes: 29f006fdefe6 ("asm-generic/atomic: Add try_cmpxchg() fallbacks") > > > > I'm not sure that this needs a fixes tag. Does anything go wrong today, or only > > later in this series? > > The patch at [1] triggered a build error in posix_acl.c/__get.acl due > to the same problem. The compilation for x86 target was OK, because > x86 defines target-specific arch_try_cmpxchg, but the compilation > broke for targets that revert to generic support. Please note that > this specific problem was recently fixed in a different way [2], but > the issue with the fallback remains. > > [1] https://lore.kernel.org/lkml/20220714173819.13312-1-ubizjak@gmail.com/ > [2] https://lore.kernel.org/lkml/20221201160103.76012-1-ubizjak@gmail.com/ > > Uros.
On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > An example is patch 10/10 from the series, which will fail without > > this fix when fallback code is used. We have: > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > where rb->head is defined as: > > > > typedef struct { > > atomic_long_t a; > > } local_t; > > > > while offset is defined as 'unsigned long'. > > Ok, but that's because we're doing the wrong thing to start with. > > Since local_t is defined in terms of atomic_long_t, we should define the > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > that in the callsite: > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) Sorry, that should be: while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) The fundamenalthing I'm trying to say is that the atomic/atomic64/atomic_long/local/local64 APIs should be type-safe, and for their try_cmpxchg() implementations, the type signature should be: ${atomictype}_try_cmpxchg(${atomictype} *ptr, ${inttype} *old, ${inttype} new) Thanks, Mark.
On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> wrote: > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > > > An example is patch 10/10 from the series, which will fail without > > > this fix when fallback code is used. We have: > > > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > > > where rb->head is defined as: > > > > > > typedef struct { > > > atomic_long_t a; > > > } local_t; > > > > > > while offset is defined as 'unsigned long'. > > > > Ok, but that's because we're doing the wrong thing to start with. > > > > Since local_t is defined in terms of atomic_long_t, we should define the > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > > that in the callsite: > > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) > > Sorry, that should be: > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) The fallbacks are a bit more complicated than above, and are different from atomic_try_cmpxchg. Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local are not defined call arch_cmpxchg_local. Also in patch 2/10, try_cmpxchg_local is introduced, where it calls arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g. : #define local_cmpxchg(l, o, n) \ (cmpxchg_local(&((l)->a.counter), (o), (n))) +#define local_try_cmpxchg(l, po, n) \ + (try_cmpxchg_local(&((l)->a.counter), (po), (n))) which is part of the local_t API. Targets should either define all these #defines, or none. There are no partial fallbacks as is the case with atomic_t. The core of the local_h API is in the local.h header. If the target doesn't define its own local.h header, then asm-generic/local.h is used that does exactly what you propose above regarding the usage of atomic functions. OTOH, when the target defines its own local.h, then the above target-dependent #define path applies. The target should define its own arch_try_cmpxchg_local, otherwise a "generic" target-dependent fallback that calls target arch_cmpxchg_local applies. In the case of x86, patch 9/10 enables new instruction by defining arch_try_cmpxchg_local. FYI, the patch sequence is carefully chosen so that x86 also exercises fallback code between different patches in the series. Targets are free to define local_t to whatever they like, but for some reason they all define it to: typedef struct { atomic_long_t a; } local_t; so they have to dig the variable out of the struct like: #define local_cmpxchg(l, o, n) \ (cmpxchg_local(&((l)->a.counter), (o), (n))) Regarding the mismatch of 'long *' vs 'unsigned long *': x86 target-specific code does for try_cmpxchg: #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \ ({ \ bool success; \ __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \ __typeof__(*(_ptr)) __old = *_old; \ __typeof__(*(_ptr)) __new = (_new); \ so, it *does* cast the "old" pointer to the type of "ptr". The generic code does *not*. This difference is dangerous, since the compilation of some code involving try_cmpxchg will compile OK for x86 but will break for other targets that use try_cmpxchg fallback templates (I was the unlucky one that tripped on this in the past). Please note that this problem is not specific to the proposed local_try_cmpxchg series, but affects the existing try_cmpxchg API. Also, I don't think that "fixing" callsites is the right thing to do. The generic code should follow x86 and cast the "old" pointer to the type of "ptr" inside the fallback. > The fundamenalthing I'm trying to say is that the > atomic/atomic64/atomic_long/local/local64 APIs should be type-safe, and for > their try_cmpxchg() implementations, the type signature should be: > > ${atomictype}_try_cmpxchg(${atomictype} *ptr, ${inttype} *old, ${inttype} new) This conversion should be performed also for the cmpxchg family of functions, if desired at all. try_cmpxchg fallback is just cmpxchg with some extra code around. Thanks, Uros.
On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote: > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > > > > > An example is patch 10/10 from the series, which will fail without > > > > this fix when fallback code is used. We have: > > > > > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > > > > > where rb->head is defined as: > > > > > > > > typedef struct { > > > > atomic_long_t a; > > > > } local_t; > > > > > > > > while offset is defined as 'unsigned long'. > > > > > > Ok, but that's because we're doing the wrong thing to start with. > > > > > > Since local_t is defined in terms of atomic_long_t, we should define the > > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > > > that in the callsite: > > > > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) > > > > Sorry, that should be: > > > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) > > The fallbacks are a bit more complicated than above, and are different > from atomic_try_cmpxchg. > > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local > are not defined call arch_cmpxchg_local. Also in patch 2/10, > try_cmpxchg_local is introduced, where it calls > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g. > : > > #define local_cmpxchg(l, o, n) \ > (cmpxchg_local(&((l)->a.counter), (o), (n))) > +#define local_try_cmpxchg(l, po, n) \ > + (try_cmpxchg_local(&((l)->a.counter), (po), (n))) > > which is part of the local_t API. Targets should either define all > these #defines, or none. There are no partial fallbacks as is the case > with atomic_t. Whether or not there are fallbacks is immaterial. In those cases, architectures can just as easily write C wrappers, e.g. long local_cmpxchg(local_t *l, long old, long new) { return cmpxchg_local(&l->a.counter, old, new); } long local_try_cmpxchg(local_t *l, long *old, long new) { return try_cmpxchg_local(&l->a.counter, old, new); } > The core of the local_h API is in the local.h header. If the target > doesn't define its own local.h header, then asm-generic/local.h is > used that does exactly what you propose above regarding the usage of > atomic functions. > > OTOH, when the target defines its own local.h, then the above > target-dependent #define path applies. The target should define its > own arch_try_cmpxchg_local, otherwise a "generic" target-dependent > fallback that calls target arch_cmpxchg_local applies. In the case of > x86, patch 9/10 enables new instruction by defining > arch_try_cmpxchg_local. > > FYI, the patch sequence is carefully chosen so that x86 also exercises > fallback code between different patches in the series. > > Targets are free to define local_t to whatever they like, but for some > reason they all define it to: > > typedef struct { > atomic_long_t a; > } local_t; Yes, which is why I used atomic_long() above. > so they have to dig the variable out of the struct like: > > #define local_cmpxchg(l, o, n) \ > (cmpxchg_local(&((l)->a.counter), (o), (n))) > > Regarding the mismatch of 'long *' vs 'unsigned long *': x86 > target-specific code does for try_cmpxchg: > > #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \ > ({ \ > bool success; \ > __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \ > __typeof__(*(_ptr)) __old = *_old; \ > __typeof__(*(_ptr)) __new = (_new); \ > > so, it *does* cast the "old" pointer to the type of "ptr". The generic > code does *not*. This difference is dangerous, since the compilation > of some code involving try_cmpxchg will compile OK for x86 but will > break for other targets that use try_cmpxchg fallback templates (I was > the unlucky one that tripped on this in the past). Please note that > this problem is not specific to the proposed local_try_cmpxchg series, > but affects the existing try_cmpxchg API. I understand the problem of arch code differing from generic code, and that we want to have *a* consistent behaviour for hte API. What I'm saying is that the behaviour we should aim for is where the 'old' pointer has a specific type (long), and we always require that, as we do for the various atomic_*() APIs of which local_*() is a cousin. > Also, I don't think that "fixing" callsites is the right thing to do. Why? What's wrong with doing that? The documentation in Documentation/core-api/local_ops.rst says: The ``local_t`` type is defined as an opaque ``signed long`` So the obvious and least surprising thing is for the local_*() functions to use 'long' for values and 'long *' for pointers to values. Requiring a cast in a few places is not the end of the world. > The generic code should follow x86 and cast the "old" pointer to the > type of "ptr" inside the fallback. Why? I disagree, and think it's far better to be strict by default. That way, accidental usage of the wrong type will be caught by the compiler, and if someone *really* wants to use a differently type then can use a cast in the callsite, which makes it really obvious when that is happening. I appreciate that may require some preparatory cleanup, but I think that's a small price to pay for having this in a clearer and more maintainable state. > > The fundamenalthing I'm trying to say is that the > > atomic/atomic64/atomic_long/local/local64 APIs should be type-safe, and for > > their try_cmpxchg() implementations, the type signature should be: > > > > ${atomictype}_try_cmpxchg(${atomictype} *ptr, ${inttype} *old, ${inttype} new) > > This conversion should be performed also for the cmpxchg family of > functions, if desired at all. try_cmpxchg fallback is just cmpxchg > with some extra code around. FWIW, I agree that we *should* make try_cmpxchg() check that ptr and old pointer are the same type. However, I don't think that's a prerequisite for doing so for local_try_cmpxchg(). Plese make local_try_cmpxchg() have a proper type-safe C prototype, as we do with the atomic*_try_cmpxchg() APIs. Thanks, Mark,
On Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@arm.com> wrote: > > On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote: > > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > > > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > > > > > > > An example is patch 10/10 from the series, which will fail without > > > > > this fix when fallback code is used. We have: > > > > > > > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > > > > > > > where rb->head is defined as: > > > > > > > > > > typedef struct { > > > > > atomic_long_t a; > > > > > } local_t; > > > > > > > > > > while offset is defined as 'unsigned long'. > > > > > > > > Ok, but that's because we're doing the wrong thing to start with. > > > > > > > > Since local_t is defined in terms of atomic_long_t, we should define the > > > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > > > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > > > > that in the callsite: > > > > > > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) > > > > > > Sorry, that should be: > > > > > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) > > > > The fallbacks are a bit more complicated than above, and are different > > from atomic_try_cmpxchg. > > > > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local > > are not defined call arch_cmpxchg_local. Also in patch 2/10, > > try_cmpxchg_local is introduced, where it calls > > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g. > > : > > > > #define local_cmpxchg(l, o, n) \ > > (cmpxchg_local(&((l)->a.counter), (o), (n))) > > +#define local_try_cmpxchg(l, po, n) \ > > + (try_cmpxchg_local(&((l)->a.counter), (po), (n))) > > > > which is part of the local_t API. Targets should either define all > > these #defines, or none. There are no partial fallbacks as is the case > > with atomic_t. > > Whether or not there are fallbacks is immaterial. > > In those cases, architectures can just as easily write C wrappers, e.g. > > long local_cmpxchg(local_t *l, long old, long new) > { > return cmpxchg_local(&l->a.counter, old, new); > } > > long local_try_cmpxchg(local_t *l, long *old, long new) > { > return try_cmpxchg_local(&l->a.counter, old, new); > } Please find attached the complete prototype patch that implements the above suggestion. The patch includes: - implementation of instrumented try_cmpxchg{,64}_local definitions - corresponding arch_try_cmpxchg{,64}_local fallback definitions - generic local{,64}_try_cmpxchg (and local{,64}_cmpxchg) C wrappers - x86 specific local_try_cmpxchg (and local_cmpxchg) C wrappers - x86 specific arch_try_cmpxchg_local definition - kernel/events/ring_buffer.c change to test local_try_cmpxchg implementation and illustrate the transition - arch/x86/events/core.c change to test local64_try_cmpxchg implementation and illustrate the transition The definition of atomic_long_t is different for 64-bit and 32-bit targets (s64 vs int), so target specific C wrappers have to use different casts to account for this difference. Uros. diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index d096b04bf80e..d9310e9363f1 100644 --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -129,13 +129,12 @@ u64 x86_perf_event_update(struct perf_event *event) * exchange a new raw count - then add that new-prev delta * count to the generic event atomically: */ -again: prev_raw_count = local64_read(&hwc->prev_count); - rdpmcl(hwc->event_base_rdpmc, new_raw_count); - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count, - new_raw_count) != prev_raw_count) - goto again; + do { + rdpmcl(hwc->event_base_rdpmc, new_raw_count); + } while (!local64_try_cmpxchg(&hwc->prev_count, &prev_raw_count, + new_raw_count)); /* * Now we have the new raw value and have updated the prev diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h index 94fbe6ae7431..540573f515b7 100644 --- a/arch/x86/include/asm/cmpxchg.h +++ b/arch/x86/include/asm/cmpxchg.h @@ -221,9 +221,15 @@ extern void __add_wrong_size(void) #define __try_cmpxchg(ptr, pold, new, size) \ __raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX) +#define __try_cmpxchg_local(ptr, pold, new, size) \ + __raw_try_cmpxchg((ptr), (pold), (new), (size), "") + #define arch_try_cmpxchg(ptr, pold, new) \ __try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr))) +#define arch_try_cmpxchg_local(ptr, pold, new) \ + __try_cmpxchg_local((ptr), (pold), (new), sizeof(*(ptr))) + /* * xadd() adds "inc" to "*ptr" and atomically returns the previous * value of "*ptr". diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h index 349a47acaa4a..d286a6c7c0b7 100644 --- a/arch/x86/include/asm/local.h +++ b/arch/x86/include/asm/local.h @@ -120,8 +120,20 @@ static inline long local_sub_return(long i, local_t *l) #define local_inc_return(l) (local_add_return(1, l)) #define local_dec_return(l) (local_sub_return(1, l)) -#define local_cmpxchg(l, o, n) \ - (cmpxchg_local(&((l)->a.counter), (o), (n))) +static inline long local_cmpxchg(local_t *l, long old, long new) +{ + return cmpxchg_local(&l->a.counter, old, new); +} + +static inline bool local_try_cmpxchg(local_t *l, long *old, long new) +{ +#ifdef CONFIG_64BIT + return try_cmpxchg_local(&l->a.counter, (s64 *)old, new); +#else + return try_cmpxchg_local(&l->a.counter, (int *)old, new); +#endif +} + /* Always has a lock prefix */ #define local_xchg(l, n) (xchg(&((l)->a.counter), (n))) diff --git a/include/asm-generic/local.h b/include/asm-generic/local.h index fca7f1d84818..7f97018df66f 100644 --- a/include/asm-generic/local.h +++ b/include/asm-generic/local.h @@ -42,6 +42,7 @@ typedef struct #define local_inc_return(l) atomic_long_inc_return(&(l)->a) #define local_cmpxchg(l, o, n) atomic_long_cmpxchg((&(l)->a), (o), (n)) +#define local_try_cmpxchg(l, po, n) atomic_long_try_cmpxchg((&(l)->a), (po), (n)) #define local_xchg(l, n) atomic_long_xchg((&(l)->a), (n)) #define local_add_unless(l, _a, u) atomic_long_add_unless((&(l)->a), (_a), (u)) #define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a) diff --git a/include/asm-generic/local64.h b/include/asm-generic/local64.h index 765be0b7d883..14963a7a6253 100644 --- a/include/asm-generic/local64.h +++ b/include/asm-generic/local64.h @@ -42,7 +42,16 @@ typedef struct { #define local64_sub_return(i, l) local_sub_return((i), (&(l)->a)) #define local64_inc_return(l) local_inc_return(&(l)->a) -#define local64_cmpxchg(l, o, n) local_cmpxchg((&(l)->a), (o), (n)) +static inline s64 local64_cmpxchg(local64_t *l, s64 old, s64 new) +{ + return local_cmpxchg(&l->a, old, new); +} + +static inline bool local64_try_cmpxchg(local64_t *l, s64 *old, s64 new) +{ + return local_try_cmpxchg(&l->a, (long *)old, new); +} + #define local64_xchg(l, n) local_xchg((&(l)->a), (n)) #define local64_add_unless(l, _a, u) local_add_unless((&(l)->a), (_a), (u)) #define local64_inc_not_zero(l) local_inc_not_zero(&(l)->a) @@ -81,6 +90,7 @@ typedef struct { #define local64_inc_return(l) atomic64_inc_return(&(l)->a) #define local64_cmpxchg(l, o, n) atomic64_cmpxchg((&(l)->a), (o), (n)) +#define local64_try_cmpxchg(l, po, n) atomic64_try_cmpxchg((&(l)->a), (po), (n)) #define local64_xchg(l, n) atomic64_xchg((&(l)->a), (n)) #define local64_add_unless(l, _a, u) atomic64_add_unless((&(l)->a), (_a), (u)) #define local64_inc_not_zero(l) atomic64_inc_not_zero(&(l)->a) diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h index 77bc5522e61c..36c92851cdee 100644 --- a/include/linux/atomic/atomic-arch-fallback.h +++ b/include/linux/atomic/atomic-arch-fallback.h @@ -217,6 +217,28 @@ #endif /* arch_try_cmpxchg64_relaxed */ +#ifndef arch_try_cmpxchg_local +#define arch_try_cmpxchg_local(_ptr, _oldp, _new) \ +({ \ + typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + ___r = arch_cmpxchg_local((_ptr), ___o, (_new)); \ + if (unlikely(___r != ___o)) \ + *___op = ___r; \ + likely(___r == ___o); \ +}) +#endif /* arch_try_cmpxchg_local */ + +#ifndef arch_try_cmpxchg64_local +#define arch_try_cmpxchg64_local(_ptr, _oldp, _new) \ +({ \ + typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + ___r = arch_cmpxchg64_local((_ptr), ___o, (_new)); \ + if (unlikely(___r != ___o)) \ + *___op = ___r; \ + likely(___r == ___o); \ +}) +#endif /* arch_try_cmpxchg64_local */ + #ifndef arch_atomic_read_acquire static __always_inline int arch_atomic_read_acquire(const atomic_t *v) @@ -2456,4 +2478,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v) #endif #endif /* _LINUX_ATOMIC_FALLBACK_H */ -// b5e87bdd5ede61470c29f7a7e4de781af3770f09 +// 1f49bd4895a4b7a5383906649027205c52ec80ab diff --git a/include/linux/atomic/atomic-instrumented.h b/include/linux/atomic/atomic-instrumented.h index 7a139ec030b0..14a9212cc987 100644 --- a/include/linux/atomic/atomic-instrumented.h +++ b/include/linux/atomic/atomic-instrumented.h @@ -2066,6 +2066,24 @@ atomic_long_dec_if_positive(atomic_long_t *v) arch_sync_cmpxchg(__ai_ptr, __VA_ARGS__); \ }) +#define try_cmpxchg_local(ptr, oldp, ...) \ +({ \ + typeof(ptr) __ai_ptr = (ptr); \ + typeof(oldp) __ai_oldp = (oldp); \ + instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \ + instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \ + arch_try_cmpxchg_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \ +}) + +#define try_cmpxchg64_local(ptr, oldp, ...) \ +({ \ + typeof(ptr) __ai_ptr = (ptr); \ + typeof(oldp) __ai_oldp = (oldp); \ + instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \ + instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \ + arch_try_cmpxchg64_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \ +}) + #define cmpxchg_double(ptr, ...) \ ({ \ typeof(ptr) __ai_ptr = (ptr); \ @@ -2083,4 +2101,4 @@ atomic_long_dec_if_positive(atomic_long_t *v) }) #endif /* _LINUX_ATOMIC_INSTRUMENTED_H */ -// 764f741eb77a7ad565dc8d99ce2837d5542e8aee +// 456e206c7e4e681126c482e4edcc6f46921ac731 diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 273a0fe7910a..111ab85ee97d 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle, perf_output_get_handle(handle); + offset = local_read(&rb->head); do { tail = READ_ONCE(rb->user_page->data_tail); - offset = head = local_read(&rb->head); + head = offset; if (!rb->overwrite) { if (unlikely(!ring_buffer_has_space(head, tail, perf_data_size(rb), @@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle, head += size; else head -= size; - } while (local_cmpxchg(&rb->head, offset, head) != offset); + } while (!local_try_cmpxchg(&rb->head, &offset, head)); if (backward) { offset = head; diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh index 3a07695e3c89..6e853f0dad8d 100755 --- a/scripts/atomic/gen-atomic-fallback.sh +++ b/scripts/atomic/gen-atomic-fallback.sh @@ -225,6 +225,10 @@ for cmpxchg in "cmpxchg" "cmpxchg64"; do gen_try_cmpxchg_fallbacks "${cmpxchg}" done +for cmpxchg in "cmpxchg_local" "cmpxchg64_local"; do + gen_try_cmpxchg_fallback "${cmpxchg}" "" +done + grep '^[a-z]' "$1" | while read name meta args; do gen_proto "${meta}" "${name}" "atomic" "int" ${args} done diff --git a/scripts/atomic/gen-atomic-instrumented.sh b/scripts/atomic/gen-atomic-instrumented.sh index 77c06526a574..c8165e9431bf 100755 --- a/scripts/atomic/gen-atomic-instrumented.sh +++ b/scripts/atomic/gen-atomic-instrumented.sh @@ -173,7 +173,7 @@ for xchg in "xchg" "cmpxchg" "cmpxchg64" "try_cmpxchg" "try_cmpxchg64"; do done done -for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg"; do +for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg" "try_cmpxchg_local" "try_cmpxchg64_local" ; do gen_xchg "${xchg}" "" "" printf "\n" done
On Tue, Apr 04, 2023 at 02:24:38PM +0200, Uros Bizjak wrote: > On Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote: > > > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > > > > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > > > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > > > > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > > > > > > > > > An example is patch 10/10 from the series, which will fail without > > > > > > this fix when fallback code is used. We have: > > > > > > > > > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > > > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > > > > > > > > > where rb->head is defined as: > > > > > > > > > > > > typedef struct { > > > > > > atomic_long_t a; > > > > > > } local_t; > > > > > > > > > > > > while offset is defined as 'unsigned long'. > > > > > > > > > > Ok, but that's because we're doing the wrong thing to start with. > > > > > > > > > > Since local_t is defined in terms of atomic_long_t, we should define the > > > > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > > > > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > > > > > that in the callsite: > > > > > > > > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) > > > > > > > > Sorry, that should be: > > > > > > > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) > > > > > > The fallbacks are a bit more complicated than above, and are different > > > from atomic_try_cmpxchg. > > > > > > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local > > > are not defined call arch_cmpxchg_local. Also in patch 2/10, > > > try_cmpxchg_local is introduced, where it calls > > > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g. > > > : > > > > > > #define local_cmpxchg(l, o, n) \ > > > (cmpxchg_local(&((l)->a.counter), (o), (n))) > > > +#define local_try_cmpxchg(l, po, n) \ > > > + (try_cmpxchg_local(&((l)->a.counter), (po), (n))) > > > > > > which is part of the local_t API. Targets should either define all > > > these #defines, or none. There are no partial fallbacks as is the case > > > with atomic_t. > > > > Whether or not there are fallbacks is immaterial. > > > > In those cases, architectures can just as easily write C wrappers, e.g. > > > > long local_cmpxchg(local_t *l, long old, long new) > > { > > return cmpxchg_local(&l->a.counter, old, new); > > } > > > > long local_try_cmpxchg(local_t *l, long *old, long new) > > { > > return try_cmpxchg_local(&l->a.counter, old, new); > > } > > Please find attached the complete prototype patch that implements the > above suggestion. > > The patch includes: > - implementation of instrumented try_cmpxchg{,64}_local definitions > - corresponding arch_try_cmpxchg{,64}_local fallback definitions > - generic local{,64}_try_cmpxchg (and local{,64}_cmpxchg) C wrappers > > - x86 specific local_try_cmpxchg (and local_cmpxchg) C wrappers > - x86 specific arch_try_cmpxchg_local definition > > - kernel/events/ring_buffer.c change to test local_try_cmpxchg > implementation and illustrate the transition > - arch/x86/events/core.c change to test local64_try_cmpxchg > implementation and illustrate the transition > > The definition of atomic_long_t is different for 64-bit and 32-bit > targets (s64 vs int), so target specific C wrappers have to use > different casts to account for this difference. > > Uros. Thanks for this! FWIW, the patch (inline below) looks good to me. Mark. > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c > index d096b04bf80e..d9310e9363f1 100644 > --- a/arch/x86/events/core.c > +++ b/arch/x86/events/core.c > @@ -129,13 +129,12 @@ u64 x86_perf_event_update(struct perf_event *event) > * exchange a new raw count - then add that new-prev delta > * count to the generic event atomically: > */ > -again: > prev_raw_count = local64_read(&hwc->prev_count); > - rdpmcl(hwc->event_base_rdpmc, new_raw_count); > > - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count, > - new_raw_count) != prev_raw_count) > - goto again; > + do { > + rdpmcl(hwc->event_base_rdpmc, new_raw_count); > + } while (!local64_try_cmpxchg(&hwc->prev_count, &prev_raw_count, > + new_raw_count)); > > /* > * Now we have the new raw value and have updated the prev > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h > index 94fbe6ae7431..540573f515b7 100644 > --- a/arch/x86/include/asm/cmpxchg.h > +++ b/arch/x86/include/asm/cmpxchg.h > @@ -221,9 +221,15 @@ extern void __add_wrong_size(void) > #define __try_cmpxchg(ptr, pold, new, size) \ > __raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX) > > +#define __try_cmpxchg_local(ptr, pold, new, size) \ > + __raw_try_cmpxchg((ptr), (pold), (new), (size), "") > + > #define arch_try_cmpxchg(ptr, pold, new) \ > __try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr))) > > +#define arch_try_cmpxchg_local(ptr, pold, new) \ > + __try_cmpxchg_local((ptr), (pold), (new), sizeof(*(ptr))) > + > /* > * xadd() adds "inc" to "*ptr" and atomically returns the previous > * value of "*ptr". > diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h > index 349a47acaa4a..d286a6c7c0b7 100644 > --- a/arch/x86/include/asm/local.h > +++ b/arch/x86/include/asm/local.h > @@ -120,8 +120,20 @@ static inline long local_sub_return(long i, local_t *l) > #define local_inc_return(l) (local_add_return(1, l)) > #define local_dec_return(l) (local_sub_return(1, l)) > > -#define local_cmpxchg(l, o, n) \ > - (cmpxchg_local(&((l)->a.counter), (o), (n))) > +static inline long local_cmpxchg(local_t *l, long old, long new) > +{ > + return cmpxchg_local(&l->a.counter, old, new); > +} > + > +static inline bool local_try_cmpxchg(local_t *l, long *old, long new) > +{ > +#ifdef CONFIG_64BIT > + return try_cmpxchg_local(&l->a.counter, (s64 *)old, new); > +#else > + return try_cmpxchg_local(&l->a.counter, (int *)old, new); > +#endif > +} > + > /* Always has a lock prefix */ > #define local_xchg(l, n) (xchg(&((l)->a.counter), (n))) > > diff --git a/include/asm-generic/local.h b/include/asm-generic/local.h > index fca7f1d84818..7f97018df66f 100644 > --- a/include/asm-generic/local.h > +++ b/include/asm-generic/local.h > @@ -42,6 +42,7 @@ typedef struct > #define local_inc_return(l) atomic_long_inc_return(&(l)->a) > > #define local_cmpxchg(l, o, n) atomic_long_cmpxchg((&(l)->a), (o), (n)) > +#define local_try_cmpxchg(l, po, n) atomic_long_try_cmpxchg((&(l)->a), (po), (n)) > #define local_xchg(l, n) atomic_long_xchg((&(l)->a), (n)) > #define local_add_unless(l, _a, u) atomic_long_add_unless((&(l)->a), (_a), (u)) > #define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a) > diff --git a/include/asm-generic/local64.h b/include/asm-generic/local64.h > index 765be0b7d883..14963a7a6253 100644 > --- a/include/asm-generic/local64.h > +++ b/include/asm-generic/local64.h > @@ -42,7 +42,16 @@ typedef struct { > #define local64_sub_return(i, l) local_sub_return((i), (&(l)->a)) > #define local64_inc_return(l) local_inc_return(&(l)->a) > > -#define local64_cmpxchg(l, o, n) local_cmpxchg((&(l)->a), (o), (n)) > +static inline s64 local64_cmpxchg(local64_t *l, s64 old, s64 new) > +{ > + return local_cmpxchg(&l->a, old, new); > +} > + > +static inline bool local64_try_cmpxchg(local64_t *l, s64 *old, s64 new) > +{ > + return local_try_cmpxchg(&l->a, (long *)old, new); > +} > + > #define local64_xchg(l, n) local_xchg((&(l)->a), (n)) > #define local64_add_unless(l, _a, u) local_add_unless((&(l)->a), (_a), (u)) > #define local64_inc_not_zero(l) local_inc_not_zero(&(l)->a) > @@ -81,6 +90,7 @@ typedef struct { > #define local64_inc_return(l) atomic64_inc_return(&(l)->a) > > #define local64_cmpxchg(l, o, n) atomic64_cmpxchg((&(l)->a), (o), (n)) > +#define local64_try_cmpxchg(l, po, n) atomic64_try_cmpxchg((&(l)->a), (po), (n)) > #define local64_xchg(l, n) atomic64_xchg((&(l)->a), (n)) > #define local64_add_unless(l, _a, u) atomic64_add_unless((&(l)->a), (_a), (u)) > #define local64_inc_not_zero(l) atomic64_inc_not_zero(&(l)->a) > diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h > index 77bc5522e61c..36c92851cdee 100644 > --- a/include/linux/atomic/atomic-arch-fallback.h > +++ b/include/linux/atomic/atomic-arch-fallback.h > @@ -217,6 +217,28 @@ > > #endif /* arch_try_cmpxchg64_relaxed */ > > +#ifndef arch_try_cmpxchg_local > +#define arch_try_cmpxchg_local(_ptr, _oldp, _new) \ > +({ \ > + typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + ___r = arch_cmpxchg_local((_ptr), ___o, (_new)); \ > + if (unlikely(___r != ___o)) \ > + *___op = ___r; \ > + likely(___r == ___o); \ > +}) > +#endif /* arch_try_cmpxchg_local */ > + > +#ifndef arch_try_cmpxchg64_local > +#define arch_try_cmpxchg64_local(_ptr, _oldp, _new) \ > +({ \ > + typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ > + ___r = arch_cmpxchg64_local((_ptr), ___o, (_new)); \ > + if (unlikely(___r != ___o)) \ > + *___op = ___r; \ > + likely(___r == ___o); \ > +}) > +#endif /* arch_try_cmpxchg64_local */ > + > #ifndef arch_atomic_read_acquire > static __always_inline int > arch_atomic_read_acquire(const atomic_t *v) > @@ -2456,4 +2478,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v) > #endif > > #endif /* _LINUX_ATOMIC_FALLBACK_H */ > -// b5e87bdd5ede61470c29f7a7e4de781af3770f09 > +// 1f49bd4895a4b7a5383906649027205c52ec80ab > diff --git a/include/linux/atomic/atomic-instrumented.h b/include/linux/atomic/atomic-instrumented.h > index 7a139ec030b0..14a9212cc987 100644 > --- a/include/linux/atomic/atomic-instrumented.h > +++ b/include/linux/atomic/atomic-instrumented.h > @@ -2066,6 +2066,24 @@ atomic_long_dec_if_positive(atomic_long_t *v) > arch_sync_cmpxchg(__ai_ptr, __VA_ARGS__); \ > }) > > +#define try_cmpxchg_local(ptr, oldp, ...) \ > +({ \ > + typeof(ptr) __ai_ptr = (ptr); \ > + typeof(oldp) __ai_oldp = (oldp); \ > + instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \ > + instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \ > + arch_try_cmpxchg_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \ > +}) > + > +#define try_cmpxchg64_local(ptr, oldp, ...) \ > +({ \ > + typeof(ptr) __ai_ptr = (ptr); \ > + typeof(oldp) __ai_oldp = (oldp); \ > + instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \ > + instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \ > + arch_try_cmpxchg64_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \ > +}) > + > #define cmpxchg_double(ptr, ...) \ > ({ \ > typeof(ptr) __ai_ptr = (ptr); \ > @@ -2083,4 +2101,4 @@ atomic_long_dec_if_positive(atomic_long_t *v) > }) > > #endif /* _LINUX_ATOMIC_INSTRUMENTED_H */ > -// 764f741eb77a7ad565dc8d99ce2837d5542e8aee > +// 456e206c7e4e681126c482e4edcc6f46921ac731 > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index 273a0fe7910a..111ab85ee97d 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle, > > perf_output_get_handle(handle); > > + offset = local_read(&rb->head); > do { > tail = READ_ONCE(rb->user_page->data_tail); > - offset = head = local_read(&rb->head); > + head = offset; > if (!rb->overwrite) { > if (unlikely(!ring_buffer_has_space(head, tail, > perf_data_size(rb), > @@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle, > head += size; > else > head -= size; > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > if (backward) { > offset = head; > diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh > index 3a07695e3c89..6e853f0dad8d 100755 > --- a/scripts/atomic/gen-atomic-fallback.sh > +++ b/scripts/atomic/gen-atomic-fallback.sh > @@ -225,6 +225,10 @@ for cmpxchg in "cmpxchg" "cmpxchg64"; do > gen_try_cmpxchg_fallbacks "${cmpxchg}" > done > > +for cmpxchg in "cmpxchg_local" "cmpxchg64_local"; do > + gen_try_cmpxchg_fallback "${cmpxchg}" "" > +done > + > grep '^[a-z]' "$1" | while read name meta args; do > gen_proto "${meta}" "${name}" "atomic" "int" ${args} > done > diff --git a/scripts/atomic/gen-atomic-instrumented.sh b/scripts/atomic/gen-atomic-instrumented.sh > index 77c06526a574..c8165e9431bf 100755 > --- a/scripts/atomic/gen-atomic-instrumented.sh > +++ b/scripts/atomic/gen-atomic-instrumented.sh > @@ -173,7 +173,7 @@ for xchg in "xchg" "cmpxchg" "cmpxchg64" "try_cmpxchg" "try_cmpxchg64"; do > done > done > > -for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg"; do > +for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg" "try_cmpxchg_local" "try_cmpxchg64_local" ; do > gen_xchg "${xchg}" "" "" > printf "\n" > done
On Tue, Apr 4, 2023 at 3:19 PM Mark Rutland <mark.rutland@arm.com> wrote: > > On Tue, Apr 04, 2023 at 02:24:38PM +0200, Uros Bizjak wrote: > > On Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote: > > > > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > > > On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote: > > > > > > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote: > > > > > > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > > > > > > > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote: > > > > > > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. > > > > > > > > > > > > > > > > Can you give an example of where we are passing an incompatible pointer? > > > > > > > > > > > > > > An example is patch 10/10 from the series, which will fail without > > > > > > > this fix when fallback code is used. We have: > > > > > > > > > > > > > > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > > > > > > > + } while (!local_try_cmpxchg(&rb->head, &offset, head)); > > > > > > > > > > > > > > where rb->head is defined as: > > > > > > > > > > > > > > typedef struct { > > > > > > > atomic_long_t a; > > > > > > > } local_t; > > > > > > > > > > > > > > while offset is defined as 'unsigned long'. > > > > > > > > > > > > Ok, but that's because we're doing the wrong thing to start with. > > > > > > > > > > > > Since local_t is defined in terms of atomic_long_t, we should define the > > > > > > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still > > > > > > have a mismatch between 'long *' and 'unsigned long *', but then we can fix > > > > > > that in the callsite: > > > > > > > > > > > > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head)) > > > > > > > > > > Sorry, that should be: > > > > > > > > > > while (!local_try_cmpxchg(&rb->head, (long *)&offset, head)) > > > > > > > > The fallbacks are a bit more complicated than above, and are different > > > > from atomic_try_cmpxchg. > > > > > > > > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local > > > > are not defined call arch_cmpxchg_local. Also in patch 2/10, > > > > try_cmpxchg_local is introduced, where it calls > > > > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g. > > > > : > > > > > > > > #define local_cmpxchg(l, o, n) \ > > > > (cmpxchg_local(&((l)->a.counter), (o), (n))) > > > > +#define local_try_cmpxchg(l, po, n) \ > > > > + (try_cmpxchg_local(&((l)->a.counter), (po), (n))) > > > > > > > > which is part of the local_t API. Targets should either define all > > > > these #defines, or none. There are no partial fallbacks as is the case > > > > with atomic_t. > > > > > > Whether or not there are fallbacks is immaterial. > > > > > > In those cases, architectures can just as easily write C wrappers, e.g. > > > > > > long local_cmpxchg(local_t *l, long old, long new) > > > { > > > return cmpxchg_local(&l->a.counter, old, new); > > > } > > > > > > long local_try_cmpxchg(local_t *l, long *old, long new) > > > { > > > return try_cmpxchg_local(&l->a.counter, old, new); > > > } > > > > Please find attached the complete prototype patch that implements the > > above suggestion. > > > > The patch includes: > > - implementation of instrumented try_cmpxchg{,64}_local definitions > > - corresponding arch_try_cmpxchg{,64}_local fallback definitions > > - generic local{,64}_try_cmpxchg (and local{,64}_cmpxchg) C wrappers > > > > - x86 specific local_try_cmpxchg (and local_cmpxchg) C wrappers > > - x86 specific arch_try_cmpxchg_local definition > > > > - kernel/events/ring_buffer.c change to test local_try_cmpxchg > > implementation and illustrate the transition > > - arch/x86/events/core.c change to test local64_try_cmpxchg > > implementation and illustrate the transition > > > > The definition of atomic_long_t is different for 64-bit and 32-bit > > targets (s64 vs int), so target specific C wrappers have to use > > different casts to account for this difference. > > > > Uros. > > Thanks for this! > > FWIW, the patch (inline below) looks good to me. Thanks, I will prepare a patch series for submission later today. Uros.
diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h index 77bc5522e61c..19debd501ee7 100644 --- a/include/linux/atomic/atomic-arch-fallback.h +++ b/include/linux/atomic/atomic-arch-fallback.h @@ -87,7 +87,7 @@ #ifndef arch_try_cmpxchg #define arch_try_cmpxchg(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -98,7 +98,7 @@ #ifndef arch_try_cmpxchg_acquire #define arch_try_cmpxchg_acquire(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg_acquire((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -109,7 +109,7 @@ #ifndef arch_try_cmpxchg_release #define arch_try_cmpxchg_release(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg_release((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -120,7 +120,7 @@ #ifndef arch_try_cmpxchg_relaxed #define arch_try_cmpxchg_relaxed(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg_relaxed((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -157,7 +157,7 @@ #ifndef arch_try_cmpxchg64 #define arch_try_cmpxchg64(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg64((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -168,7 +168,7 @@ #ifndef arch_try_cmpxchg64_acquire #define arch_try_cmpxchg64_acquire(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg64_acquire((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -179,7 +179,7 @@ #ifndef arch_try_cmpxchg64_release #define arch_try_cmpxchg64_release(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg64_release((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -190,7 +190,7 @@ #ifndef arch_try_cmpxchg64_relaxed #define arch_try_cmpxchg64_relaxed(_ptr, _oldp, _new) \ ({ \ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \ ___r = arch_cmpxchg64_relaxed((_ptr), ___o, (_new)); \ if (unlikely(___r != ___o)) \ *___op = ___r; \ @@ -2456,4 +2456,4 @@ arch_atomic64_dec_if_positive(atomic64_t *v) #endif #endif /* _LINUX_ATOMIC_FALLBACK_H */ -// b5e87bdd5ede61470c29f7a7e4de781af3770f09 +// 1b4d4c82ae653389cd1538d5b07170267d9b3837 diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh index 3a07695e3c89..39f447161108 100755 --- a/scripts/atomic/gen-atomic-fallback.sh +++ b/scripts/atomic/gen-atomic-fallback.sh @@ -171,7 +171,7 @@ cat <<EOF #ifndef arch_try_${cmpxchg}${order} #define arch_try_${cmpxchg}${order}(_ptr, _oldp, _new) \\ ({ \\ - typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \\ + typeof(*(_ptr)) *___op = (typeof(_ptr))(_oldp), ___o = *___op, ___r; \\ ___r = arch_${cmpxchg}${order}((_ptr), ___o, (_new)); \\ if (unlikely(___r != ___o)) \\ *___op = ___r; \\
Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning. Fixes: 29f006fdefe6 ("asm-generic/atomic: Add try_cmpxchg() fallbacks") Cc: Will Deacon <will@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- include/linux/atomic/atomic-arch-fallback.h | 18 +++++++++--------- scripts/atomic/gen-atomic-fallback.sh | 2 +- 2 files changed, 10 insertions(+), 10 deletions(-)