| Message ID | 20190618211440.54179-1-mka@chromium.org |
|---|---|
| State | Accepted |
| Delegated to: | David Miller |
| Headers | show |
| Series | net/ipv4: fib_trie: Avoid cryptic ternary expressions | expand |
Hi, On Tue, Jun 18, 2019 at 2:14 PM Matthias Kaehlcke <mka@chromium.org> wrote: > > empty_child_inc/dec() use the ternary operator for conditional > operations. The conditions involve the post/pre in/decrement > operator and the operation is only performed when the condition > is *not* true. This is hard to parse for humans, use a regular > 'if' construct instead and perform the in/decrement separately. > > This also fixes two warnings that are emitted about the value > of the ternary expression being unused, when building the kernel > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > (https://lore.kernel.org/patchwork/patch/1089869/): > > CC net/ipv4/fib_trie.o > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org> > --- > I have no good understanding of the fib_trie code, but the > disentangled code looks wrong, and it should be equivalent to the > cryptic version, unless I messed it up. In empty_child_inc() > 'full_children' is only incremented when 'empty_children' is -1. I > suspect a bug in the cryptic code, but am surprised why it hasn't > blown up yet. Or is it intended behavior that is just > super-counterintuitive? > > For now I'm leaving it at disentangling the cryptic expressions, > if there is a bug we can discuss what action to take. > --- > net/ipv4/fib_trie.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) I have no knowledge of this code either but Matthias's patch looks sane to me and I agree with the disentangling before making functional changes. My own personal belief is that this is pointing out a bug somewhere. Since "empty_children" ends up being an unsigned type it doesn't feel like it was by-design that -1 is ever a value that should be in there. In any case: Reviewed-by: Douglas Anderson <dianders@chromium.org>
On Tue, Jun 18, 2019 at 02:45:34PM -0700, Doug Anderson wrote: > Hi, > > On Tue, Jun 18, 2019 at 2:14 PM Matthias Kaehlcke <mka@chromium.org> wrote: > > > > empty_child_inc/dec() use the ternary operator for conditional > > operations. The conditions involve the post/pre in/decrement > > operator and the operation is only performed when the condition > > is *not* true. This is hard to parse for humans, use a regular > > 'if' construct instead and perform the in/decrement separately. > > > > This also fixes two warnings that are emitted about the value > > of the ternary expression being unused, when building the kernel > > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > > (https://lore.kernel.org/patchwork/patch/1089869/): > > > > CC net/ipv4/fib_trie.o > > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org> > > --- > > I have no good understanding of the fib_trie code, but the > > disentangled code looks wrong, and it should be equivalent to the > > cryptic version, unless I messed it up. In empty_child_inc() > > 'full_children' is only incremented when 'empty_children' is -1. I > > suspect a bug in the cryptic code, but am surprised why it hasn't > > blown up yet. Or is it intended behavior that is just > > super-counterintuitive? > > > > For now I'm leaving it at disentangling the cryptic expressions, > > if there is a bug we can discuss what action to take. > > --- > > net/ipv4/fib_trie.c | 10 ++++++++-- > > 1 file changed, 8 insertions(+), 2 deletions(-) > > I have no knowledge of this code either but Matthias's patch looks > sane to me and I agree with the disentangling before making functional > changes. I in terms of the -stable process it might make sense to either disentangle & fix in a single step, or first fix the cryptic code (shudder!) and then disentangle it. I guess if we make it a series disentangle & fix could be separate steps. I'm open to whatever maintainers & stable folks prefer. > My own personal belief is that this is pointing out a bug somewhere. > Since "empty_children" ends up being an unsigned type it doesn't feel > like it was by-design that -1 is ever a value that should be in there. good point that 'empty_children' is unsigned, that indeed reinforces the bug theory. > In any case: > > Reviewed-by: Douglas Anderson <dianders@chromium.org> Thanks!
On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke wrote: > empty_child_inc/dec() use the ternary operator for conditional > operations. The conditions involve the post/pre in/decrement > operator and the operation is only performed when the condition > is *not* true. This is hard to parse for humans, use a regular > 'if' construct instead and perform the in/decrement separately. > > This also fixes two warnings that are emitted about the value > of the ternary expression being unused, when building the kernel > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > (https://lore.kernel.org/patchwork/patch/1089869/): > > CC net/ipv4/fib_trie.o > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > As an FYI, this is also being fixed in clang: https://bugs.llvm.org/show_bug.cgi?id=42239 https://reviews.llvm.org/D63369 > Signed-off-by: Matthias Kaehlcke <mka@chromium.org> > --- > I have no good understanding of the fib_trie code, but the > disentangled code looks wrong, and it should be equivalent to the > cryptic version, unless I messed it up. In empty_child_inc() > 'full_children' is only incremented when 'empty_children' is -1. I > suspect a bug in the cryptic code, but am surprised why it hasn't > blown up yet. Or is it intended behavior that is just > super-counterintuitive? > > For now I'm leaving it at disentangling the cryptic expressions, > if there is a bug we can discuss what action to take. > --- > net/ipv4/fib_trie.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c > index 868c74771fa9..cf7788e336b7 100644 > --- a/net/ipv4/fib_trie.c > +++ b/net/ipv4/fib_trie.c > @@ -338,12 +338,18 @@ static struct tnode *tnode_alloc(int bits) > > static inline void empty_child_inc(struct key_vector *n) > { > - ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > + tn_info(n)->empty_children++; > + > + if (!tn_info(n)->empty_children) > + tn_info(n)->full_children++; > } > > static inline void empty_child_dec(struct key_vector *n) > { > - tn_info(n)->empty_children-- ? : tn_info(n)->full_children--; > + if (!tn_info(n)->empty_children) > + tn_info(n)->full_children--; > + > + tn_info(n)->empty_children--; > } > > static struct key_vector *leaf_new(t_key key, struct fib_alias *fa) > -- > 2.22.0.410.gd8fdbe21b5-goog >
On Tue, Jun 18, 2019 at 04:04:20PM -0700, Nathan Chancellor wrote: > On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke wrote: > > empty_child_inc/dec() use the ternary operator for conditional > > operations. The conditions involve the post/pre in/decrement > > operator and the operation is only performed when the condition > > is *not* true. This is hard to parse for humans, use a regular > > 'if' construct instead and perform the in/decrement separately. > > > > This also fixes two warnings that are emitted about the value > > of the ternary expression being unused, when building the kernel > > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > > (https://lore.kernel.org/patchwork/patch/1089869/): > > > > CC net/ipv4/fib_trie.o > > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > > > As an FYI, this is also being fixed in clang: > > https://bugs.llvm.org/show_bug.cgi?id=42239 > > https://reviews.llvm.org/D63369 Great, thanks! In this case it was actually useful to get the warning, even though it didn't point out the actual bug. I think in general it would be preferable to avoid such constructs, even when they are correct. But then again, it's the reviewers/maintainers task to avoid unnecessarily cryptic code from slipping in, and this just happens to be one instance where the compiler could have helped.
On Tue, Jun 18, 2019 at 4:04 PM Nathan Chancellor <natechancellor@gmail.com> wrote: > > On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke wrote: > > empty_child_inc/dec() use the ternary operator for conditional > > operations. The conditions involve the post/pre in/decrement > > operator and the operation is only performed when the condition > > is *not* true. This is hard to parse for humans, use a regular > > 'if' construct instead and perform the in/decrement separately. > > > > This also fixes two warnings that are emitted about the value > > of the ternary expression being unused, when building the kernel > > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > > (https://lore.kernel.org/patchwork/patch/1089869/): > > > > CC net/ipv4/fib_trie.o > > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > > > As an FYI, this is also being fixed in clang: > > https://bugs.llvm.org/show_bug.cgi?id=42239 > > https://reviews.llvm.org/D63369 While I'm glad we found and fixed a bug in Clang; I'm still for fixing the underlying code from a readability perspective. If it takes longer than a few seconds to understand a statement to the non-author, the code as written may be too clever. As a side note, I'm going to try to see if MAINTAINERS and scripts/get_maintainers.pl supports regexes on the commit messages in order to cc our mailing list (clang-built-linux <clang-built-linux@googlegroups.com>) automatically; but in the meantime please try to remember to cc the list manually. I usually find it quickly from the bookmarked https://clangbuiltlinux.github.io/ which lists it there) (but I should still automate this). > > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org> > > --- > > I have no good understanding of the fib_trie code, but the > > disentangled code looks wrong, and it should be equivalent to the > > cryptic version, unless I messed it up. In empty_child_inc() > > 'full_children' is only incremented when 'empty_children' is -1. I > > suspect a bug in the cryptic code, but am surprised why it hasn't > > blown up yet. Or is it intended behavior that is just > > super-counterintuitive? > > > > For now I'm leaving it at disentangling the cryptic expressions, > > if there is a bug we can discuss what action to take. > > --- > > net/ipv4/fib_trie.c | 10 ++++++++-- > > 1 file changed, 8 insertions(+), 2 deletions(-) > > > > diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c > > index 868c74771fa9..cf7788e336b7 100644 > > --- a/net/ipv4/fib_trie.c > > +++ b/net/ipv4/fib_trie.c > > @@ -338,12 +338,18 @@ static struct tnode *tnode_alloc(int bits) > > > > static inline void empty_child_inc(struct key_vector *n) > > { > > - ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > + tn_info(n)->empty_children++; > > + > > + if (!tn_info(n)->empty_children) > > + tn_info(n)->full_children++; > > } > > > > static inline void empty_child_dec(struct key_vector *n) > > { > > - tn_info(n)->empty_children-- ? : tn_info(n)->full_children--; > > + if (!tn_info(n)->empty_children) > > + tn_info(n)->full_children--; > > + > > + tn_info(n)->empty_children--; This change looks correct to me, so thanks for the patch and: Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> (Bikeshed; I prefer pre-inc/dec to post-inc/dec unless you really care about the previous value. Should be irrelevant with a sufficiently smart compiler though. ;) )
On Tue, Jun 18, 2019 at 4:22 PM Matthias Kaehlcke <mka@chromium.org> wrote: > > On Tue, Jun 18, 2019 at 04:04:20PM -0700, Nathan Chancellor wrote: > > On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke wrote: > > > empty_child_inc/dec() use the ternary operator for conditional > > > operations. The conditions involve the post/pre in/decrement > > > operator and the operation is only performed when the condition > > > is *not* true. This is hard to parse for humans, use a regular > > > 'if' construct instead and perform the in/decrement separately. > > > > > > This also fixes two warnings that are emitted about the value > > > of the ternary expression being unused, when building the kernel > > > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > > > (https://lore.kernel.org/patchwork/patch/1089869/): > > > > > > CC net/ipv4/fib_trie.o > > > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > > > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > > > > > > As an FYI, this is also being fixed in clang: > > > > https://bugs.llvm.org/show_bug.cgi?id=42239 > > > > https://reviews.llvm.org/D63369 > > Great, thanks! > > In this case it was actually useful to get the warning, even though it > didn't point out the actual bug. I think in general it would be > preferable to avoid such constructs, even when they are correct. But > then again, it's the reviewers/maintainers task to avoid unnecessarily > cryptic code from slipping in, and this just happens to be one instance > where the compiler could have helped. So it took me a bit to remember/understand it as well since I haven't touched the code in over 4 years, however part of that is because the comment for this code is actually buried down in put_child. Essentially this is just meant to be an add w/ carry and a sub w/ borrow to address a potential overflow if bits == KEYLENGTH. If you want you can add: Fixes: 95f60ea3e99a ("fib_trie: Add collapse() and should_collapse() to resize") Acked-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
On Tue, 2019-06-18 at 16:23 -0700, Nick Desaulniers wrote: > As a side note, I'm going to try to see if MAINTAINERS and > scripts/get_maintainers.pl supports regexes on the commit messages in > order to cc our mailing list Neither. Why should either?
On Wed, Jun 19, 2019 at 2:36 AM Joe Perches <joe@perches.com> wrote: > > On Tue, 2019-06-18 at 16:23 -0700, Nick Desaulniers wrote: > > As a side note, I'm going to try to see if MAINTAINERS and > > scripts/get_maintainers.pl supports regexes on the commit messages in > > order to cc our mailing list > > Neither. Why should either? Looks like `K:` is exactly what I'm looking for. Joe, how does: https://github.com/ClangBuiltLinux/linux/commit/a0a64b8d65c4e7e033f49e48cc610d6e4002927e look? Is there a maintainer for MAINTAINERS or do I just send the patch to Linus?
On Wed, 2019-06-19 at 10:41 -0700, Nick Desaulniers wrote: > On Wed, Jun 19, 2019 at 2:36 AM Joe Perches <joe@perches.com> wrote: > > On Tue, 2019-06-18 at 16:23 -0700, Nick Desaulniers wrote: > > > As a side note, I'm going to try to see if MAINTAINERS and > > > scripts/get_maintainers.pl supports regexes on the commit messages in > > > order to cc our mailing list > > > > Neither. Why should either? > > Looks like `K:` is exactly what I'm looking for. Using K: for commit message content isn't the intended use, but if it works for you, fine by me. > Joe, how does: > https://github.com/ClangBuiltLinux/linux/commit/a0a64b8d65c4e7e033f49e48cc610d6e4002927e > look? You might consider using K: \b(?i:clang|llvm)\b to get case insensitive matches. > Is there a maintainer for MAINTAINERS or do I just send the > patch to Linus? Generally MAINTAINER patches go via Andrew Morton or indirectly along with other changes via a pull request to Linus.
From: Matthias Kaehlcke <mka@chromium.org> Date: Tue, 18 Jun 2019 14:14:40 -0700 > empty_child_inc/dec() use the ternary operator for conditional > operations. The conditions involve the post/pre in/decrement > operator and the operation is only performed when the condition > is *not* true. This is hard to parse for humans, use a regular > 'if' construct instead and perform the in/decrement separately. > > This also fixes two warnings that are emitted about the value > of the ternary expression being unused, when building the kernel > with clang + "kbuild: Remove unnecessary -Wno-unused-value" > (https://lore.kernel.org/patchwork/patch/1089869/): > > CC net/ipv4/fib_trie.o > net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] > ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org> Applied to net-next, thanks.
diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c index 868c74771fa9..cf7788e336b7 100644 --- a/net/ipv4/fib_trie.c +++ b/net/ipv4/fib_trie.c @@ -338,12 +338,18 @@ static struct tnode *tnode_alloc(int bits) static inline void empty_child_inc(struct key_vector *n) { - ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; + tn_info(n)->empty_children++; + + if (!tn_info(n)->empty_children) + tn_info(n)->full_children++; } static inline void empty_child_dec(struct key_vector *n) { - tn_info(n)->empty_children-- ? : tn_info(n)->full_children--; + if (!tn_info(n)->empty_children) + tn_info(n)->full_children--; + + tn_info(n)->empty_children--; } static struct key_vector *leaf_new(t_key key, struct fib_alias *fa)
empty_child_inc/dec() use the ternary operator for conditional operations. The conditions involve the post/pre in/decrement operator and the operation is only performed when the condition is *not* true. This is hard to parse for humans, use a regular 'if' construct instead and perform the in/decrement separately. This also fixes two warnings that are emitted about the value of the ternary expression being unused, when building the kernel with clang + "kbuild: Remove unnecessary -Wno-unused-value" (https://lore.kernel.org/patchwork/patch/1089869/): CC net/ipv4/fib_trie.o net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value] ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children; Signed-off-by: Matthias Kaehlcke <mka@chromium.org> --- I have no good understanding of the fib_trie code, but the disentangled code looks wrong, and it should be equivalent to the cryptic version, unless I messed it up. In empty_child_inc() 'full_children' is only incremented when 'empty_children' is -1. I suspect a bug in the cryptic code, but am surprised why it hasn't blown up yet. Or is it intended behavior that is just super-counterintuitive? For now I'm leaving it at disentangling the cryptic expressions, if there is a bug we can discuss what action to take. --- net/ipv4/fib_trie.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)