Message ID | 20190911164202.31136-2-dgilbert@redhat.com |
---|---|
State | New |
Headers | show |
Series | Automatic RCU read unlock | expand |
On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > g_auto infrastrcture (and thus whatever the compilers hooks are) to > release it on all exits of the block. > > Note this macro has a variable declaration in, and hence is not in > a while loop. > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > --- > include/qemu/rcu.h | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > index 22876d1428..6a25b27d28 100644 > --- a/include/qemu/rcu.h > +++ b/include/qemu/rcu.h > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > }), \ > (RCUCBFunc *)g_free); > > +typedef char rcu_read_auto_t; > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > +{ > + rcu_read_unlock(); > +} > + > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > + _rcu_read_auto = 'x'; \ > + rcu_read_lock(); > + Functionally this works, but my gut feeling would be to follow the design of GMutexLocker as-is: https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new so you get a use pattern of g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); This makes it explicit that the code is creating a variable here, which in turns means it is clear to force unlock early with g_clear_pointer(&locker, rcu_read_locker_free) Regards, Daniel
* Daniel P. Berrangé (berrange@redhat.com) wrote: > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > release it on all exits of the block. > > > > Note this macro has a variable declaration in, and hence is not in > > a while loop. > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > --- > > include/qemu/rcu.h | 12 ++++++++++++ > > 1 file changed, 12 insertions(+) > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > index 22876d1428..6a25b27d28 100644 > > --- a/include/qemu/rcu.h > > +++ b/include/qemu/rcu.h > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > }), \ > > (RCUCBFunc *)g_free); > > > > +typedef char rcu_read_auto_t; > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > +{ > > + rcu_read_unlock(); > > +} > > + > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > + _rcu_read_auto = 'x'; \ > > + rcu_read_lock(); > > + > > Functionally this works, but my gut feeling would be to follow > the design of GMutexLocker as-is: > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > so you get a use pattern of > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > This makes it explicit that the code is creating a variable here, which > in turns means it is clear to force unlock early with > > g_clear_pointer(&locker, rcu_read_locker_free) The difference compared to the g-mutex-locker is that I don't have another object to use as my pointer; that uses the address of the GMutex as the dummy pointer value. I did try an experiment with g_autoptr and found that it did need to return a non-NULL value for it to work, which then lead me to think what value to use - while it seems to work if I return (void *)1 it makes me nervous. Dave > > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > > release it on all exits of the block. > > > > > > Note this macro has a variable declaration in, and hence is not in > > > a while loop. > > > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > --- > > > include/qemu/rcu.h | 12 ++++++++++++ > > > 1 file changed, 12 insertions(+) > > > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > > index 22876d1428..6a25b27d28 100644 > > > --- a/include/qemu/rcu.h > > > +++ b/include/qemu/rcu.h > > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > > }), \ > > > (RCUCBFunc *)g_free); > > > > > > +typedef char rcu_read_auto_t; > > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > > +{ > > > + rcu_read_unlock(); > > > +} > > > + > > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > > + _rcu_read_auto = 'x'; \ > > > + rcu_read_lock(); > > > + > > > > Functionally this works, but my gut feeling would be to follow > > the design of GMutexLocker as-is: > > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > > > so you get a use pattern of > > > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > > > This makes it explicit that the code is creating a variable here, which > > in turns means it is clear to force unlock early with > > > > g_clear_pointer(&locker, rcu_read_locker_free) > > The difference compared to the g-mutex-locker is that I don't have > another object to use as my pointer; that uses the address of the GMutex > as the dummy pointer value. I did try an experiment with g_autoptr > and found that it did need to return a non-NULL value for it to work, > which then lead me to think what value to use - while it seems to work > if I return (void *)1 it makes me nervous. Yeah, '(void*)1' would have been what I'd pick. The only thing that the value is used for is to pass to the rcu_read_locker_free() function which ignores it, which seems safe enough. Regards, Daniel
* Daniel P. Berrangé (berrange@redhat.com) wrote: > On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > > > release it on all exits of the block. > > > > > > > > Note this macro has a variable declaration in, and hence is not in > > > > a while loop. > > > > > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > > --- > > > > include/qemu/rcu.h | 12 ++++++++++++ > > > > 1 file changed, 12 insertions(+) > > > > > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > > > index 22876d1428..6a25b27d28 100644 > > > > --- a/include/qemu/rcu.h > > > > +++ b/include/qemu/rcu.h > > > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > > > }), \ > > > > (RCUCBFunc *)g_free); > > > > > > > > +typedef char rcu_read_auto_t; > > > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > > > +{ > > > > + rcu_read_unlock(); > > > > +} > > > > + > > > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > > > > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > > > + _rcu_read_auto = 'x'; \ > > > > + rcu_read_lock(); > > > > + > > > > > > Functionally this works, but my gut feeling would be to follow > > > the design of GMutexLocker as-is: > > > > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > > > > > so you get a use pattern of > > > > > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > > > > > This makes it explicit that the code is creating a variable here, which > > > in turns means it is clear to force unlock early with > > > > > > g_clear_pointer(&locker, rcu_read_locker_free) > > > > The difference compared to the g-mutex-locker is that I don't have > > another object to use as my pointer; that uses the address of the GMutex > > as the dummy pointer value. I did try an experiment with g_autoptr > > and found that it did need to return a non-NULL value for it to work, > > which then lead me to think what value to use - while it seems to work > > if I return (void *)1 it makes me nervous. > > Yeah, '(void*)1' would have been what I'd pick. The only thing that the > value is used for is to pass to the rcu_read_locker_free() function > which ignores it, which seems safe enough. glib seems to be at least checking it; if you pass NULL the free'r doesn't get called; so it worries me that we'd be relying on the current definition. Dave > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On Wed, Sep 11, 2019 at 06:10:28PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrote: > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > > > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > > > > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > > > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > > > > release it on all exits of the block. > > > > > > > > > > Note this macro has a variable declaration in, and hence is not in > > > > > a while loop. > > > > > > > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > > > --- > > > > > include/qemu/rcu.h | 12 ++++++++++++ > > > > > 1 file changed, 12 insertions(+) > > > > > > > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > > > > index 22876d1428..6a25b27d28 100644 > > > > > --- a/include/qemu/rcu.h > > > > > +++ b/include/qemu/rcu.h > > > > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > > > > }), \ > > > > > (RCUCBFunc *)g_free); > > > > > > > > > > +typedef char rcu_read_auto_t; > > > > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > > > > +{ > > > > > + rcu_read_unlock(); > > > > > +} > > > > > + > > > > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > > > > > > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > > > > + _rcu_read_auto = 'x'; \ > > > > > + rcu_read_lock(); > > > > > + > > > > > > > > Functionally this works, but my gut feeling would be to follow > > > > the design of GMutexLocker as-is: > > > > > > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > > > > > > > so you get a use pattern of > > > > > > > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > > > > > > > This makes it explicit that the code is creating a variable here, which > > > > in turns means it is clear to force unlock early with > > > > > > > > g_clear_pointer(&locker, rcu_read_locker_free) > > > > > > The difference compared to the g-mutex-locker is that I don't have > > > another object to use as my pointer; that uses the address of the GMutex > > > as the dummy pointer value. I did try an experiment with g_autoptr > > > and found that it did need to return a non-NULL value for it to work, > > > which then lead me to think what value to use - while it seems to work > > > if I return (void *)1 it makes me nervous. > > > > Yeah, '(void*)1' would have been what I'd pick. The only thing that the > > value is used for is to pass to the rcu_read_locker_free() function > > which ignores it, which seems safe enough. > > glib seems to be at least checking it; if you pass NULL the free'r > doesn't get called; so it worries me that we'd be relying on the current > definition. This NULL check is part of the API semantics defined for G_DEFINE_AUTO_CLEANUO_FREE_FUNC. It lets you define what the "empty" value is, typically 'NULL', but in fact you don't need to use a pointer type at all. You can use an 'int', for example, and declare that '-1' is your "empty" value: https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#G-DEFINE-AUTO-CLEANUP-FREE-FUNC:CAPS Regards, Daniel
* Daniel P. Berrangé (berrange@redhat.com) wrote: > On Wed, Sep 11, 2019 at 06:10:28PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrote: > > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > > > > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > > > > > > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > > > > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > > > > > release it on all exits of the block. > > > > > > > > > > > > Note this macro has a variable declaration in, and hence is not in > > > > > > a while loop. > > > > > > > > > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > > > > --- > > > > > > include/qemu/rcu.h | 12 ++++++++++++ > > > > > > 1 file changed, 12 insertions(+) > > > > > > > > > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > > > > > index 22876d1428..6a25b27d28 100644 > > > > > > --- a/include/qemu/rcu.h > > > > > > +++ b/include/qemu/rcu.h > > > > > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > > > > > }), \ > > > > > > (RCUCBFunc *)g_free); > > > > > > > > > > > > +typedef char rcu_read_auto_t; > > > > > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > > > > > +{ > > > > > > + rcu_read_unlock(); > > > > > > +} > > > > > > + > > > > > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > > > > > > > > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > > > > > + _rcu_read_auto = 'x'; \ > > > > > > + rcu_read_lock(); > > > > > > + > > > > > > > > > > Functionally this works, but my gut feeling would be to follow > > > > > the design of GMutexLocker as-is: > > > > > > > > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > > > > > > > > > so you get a use pattern of > > > > > > > > > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > > > > > > > > > This makes it explicit that the code is creating a variable here, which > > > > > in turns means it is clear to force unlock early with > > > > > > > > > > g_clear_pointer(&locker, rcu_read_locker_free) > > > > > > > > The difference compared to the g-mutex-locker is that I don't have > > > > another object to use as my pointer; that uses the address of the GMutex > > > > as the dummy pointer value. I did try an experiment with g_autoptr > > > > and found that it did need to return a non-NULL value for it to work, > > > > which then lead me to think what value to use - while it seems to work > > > > if I return (void *)1 it makes me nervous. > > > > > > Yeah, '(void*)1' would have been what I'd pick. The only thing that the > > > value is used for is to pass to the rcu_read_locker_free() function > > > which ignores it, which seems safe enough. > > > > glib seems to be at least checking it; if you pass NULL the free'r > > doesn't get called; so it worries me that we'd be relying on the current > > definition. > > This NULL check is part of the API semantics defined for > G_DEFINE_AUTO_CLEANUO_FREE_FUNC. It lets you define > what the "empty" value is, typically 'NULL', but > in fact you don't need to use a pointer type at all. You > can use an 'int', for example, and declare that '-1' > is your "empty" value: > > https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#G-DEFINE-AUTO-CLEANUP-FREE-FUNC:CAPS Ah OK, yep that makes sense; I'll flip it around. Dave > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On 9/11/19 11:42 AM, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > g_auto infrastrcture (and thus whatever the compilers hooks are) to > release it on all exits of the block. > > Note this macro has a variable declaration in, and hence is not in > a while loop. > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > --- > include/qemu/rcu.h | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > index 22876d1428..6a25b27d28 100644 > --- a/include/qemu/rcu.h > +++ b/include/qemu/rcu.h > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > }), \ > (RCUCBFunc *)g_free); > > +typedef char rcu_read_auto_t; Declaring new types ending in _t collides with the namespace reserved by POSIX. While I don't think it will bite us, it's still worth considering if a different name is better. > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > +{ > + rcu_read_unlock(); > +} > + > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > + > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > + _rcu_read_auto = 'x'; \ I'm a bit lost at where _rcu_read_auto is declared. (I could understand if an earlier macro had created that typedef via concatenating _ with rcu_read_auto_t, but making the preprocessor drop _t is not possible. Is this a typo, and if so, why did the compiler not complain?) > + rcu_read_lock(); > + > #ifdef __cplusplus > } > #endif >
On 9/11/19 12:40 PM, Eric Blake wrote: >> + >> +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ >> + _rcu_read_auto = 'x'; \ > > I'm a bit lost at where _rcu_read_auto is declared. (I could understand > if an earlier macro had created that typedef via concatenating _ with > rcu_read_auto_t, but making the preprocessor drop _t is not possible. Is > this a typo, and if so, why did the compiler not complain?) Okay, I read it wrong. This rendering would be easier for me to understand (you are declaring a dummy variable right here): #define RCU_READ_LOCK_AUTO \ g_auto(rcu_read_auto_t) _rcu_read_auto = 'x'; \ ... In other words, I'm not used to expecting a split between type and variable name across two lines, especially when the type is itself a macro call, and where my first reading didn't spot that (rcu_read_auto_t) was not the name of the argument to a mixed-case macro RCU_READ_LOACK_AUTO_g_auto, rather than g_auto(...) being the start of the parameter-less macro RCU_READ_LOCK_AUTO definition.
On 9/11/19 11:56 AM, Daniel P. Berrangé wrote: > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: >> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> >> >> RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's spurious double space >> g_auto infrastrcture (and thus whatever the compilers hooks are) to infrastructure compiler's >> release it on all exits of the block. >> >> Note this macro has a variable declaration in, and hence is not in >> a while loop. >> >> +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ >> + _rcu_read_auto = 'x'; \ >> + rcu_read_lock(); >> + > > Functionally this works, but my gut feeling would be to follow > the design of GMutexLocker as-is: > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > so you get a use pattern of > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); Another pattern to consider: nbdkit uses: #define ACQUIRE_LOCK_FOR_CURRENT_SCOPE(mutex) \ CLEANUP_UNLOCK pthread_mutex_t *_lock = mutex; \ do { \ int _r = pthread_mutex_lock (_lock); \ assert (!_r); \ } while (0) with later code calling: ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > > This makes it explicit that the code is creating a variable here, which > in turns means it is clear to force unlock early with > > g_clear_pointer(&locker, rcu_read_locker_free) Yes, this aspect of glib is nicer than the corresponding nbdkit usage pattern.
* Eric Blake (eblake@redhat.com) wrote: > On 9/11/19 12:40 PM, Eric Blake wrote: > > >> + > >> +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > >> + _rcu_read_auto = 'x'; \ > > > > I'm a bit lost at where _rcu_read_auto is declared. (I could understand > > if an earlier macro had created that typedef via concatenating _ with > > rcu_read_auto_t, but making the preprocessor drop _t is not possible. Is > > this a typo, and if so, why did the compiler not complain?) > > Okay, I read it wrong. This rendering would be easier for me to > understand (you are declaring a dummy variable right here): > > #define RCU_READ_LOCK_AUTO \ > g_auto(rcu_read_auto_t) _rcu_read_auto = 'x'; \ > ... > > In other words, I'm not used to expecting a split between type and > variable name across two lines, especially when the type is itself a > macro call, and where my first reading didn't spot that > (rcu_read_auto_t) was not the name of the argument to a mixed-case macro > RCU_READ_LOACK_AUTO_g_auto, rather than g_auto(...) being the start of > the parameter-less macro RCU_READ_LOCK_AUTO definition. Yep, that's simplified after the rework Dan suggested. Dave > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3226 > Virtualization: qemu.org | libvirt.org > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
* Eric Blake (eblake@redhat.com) wrote: > On 9/11/19 11:56 AM, Daniel P. Berrangé wrote: > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (git) wrote: > >> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > >> > >> RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > spurious double space > > >> g_auto infrastrcture (and thus whatever the compilers hooks are) to > > infrastructure > compiler's Thanks. Dave > >> release it on all exits of the block. > >> > >> Note this macro has a variable declaration in, and hence is not in > >> a while loop. > >> > > >> +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > >> + _rcu_read_auto = 'x'; \ > >> + rcu_read_lock(); > >> + > > > > Functionally this works, but my gut feeling would be to follow > > the design of GMutexLocker as-is: > > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex-locker-new > > > > so you get a use pattern of > > > > g_autoptr(rcu_read_locker) locker = rcu_read_locker_new(); > > Another pattern to consider: nbdkit uses: > > #define ACQUIRE_LOCK_FOR_CURRENT_SCOPE(mutex) \ > CLEANUP_UNLOCK pthread_mutex_t *_lock = mutex; \ > do { \ > int _r = pthread_mutex_lock (_lock); \ > assert (!_r); \ > } while (0) > > with later code calling: > > ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); > > > > > This makes it explicit that the code is creating a variable here, which > > in turns means it is clear to force unlock early with > > > > g_clear_pointer(&locker, rcu_read_locker_free) > > Yes, this aspect of glib is nicer than the corresponding nbdkit usage > pattern. > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3226 > Virtualization: qemu.org | libvirt.org > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
* Eric Blake (eblake@redhat.com) wrote: > On 9/11/19 11:42 AM, Dr. David Alan Gilbert (git) wrote: > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > g_auto infrastrcture (and thus whatever the compilers hooks are) to > > release it on all exits of the block. > > > > Note this macro has a variable declaration in, and hence is not in > > a while loop. > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > --- > > include/qemu/rcu.h | 12 ++++++++++++ > > 1 file changed, 12 insertions(+) > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > index 22876d1428..6a25b27d28 100644 > > --- a/include/qemu/rcu.h > > +++ b/include/qemu/rcu.h > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); > > }), \ > > (RCUCBFunc *)g_free); > > > > +typedef char rcu_read_auto_t; > > Declaring new types ending in _t collides with the namespace reserved by > POSIX. While I don't think it will bite us, it's still worth > considering if a different name is better. Thanks, I've renamed it to 'RCUReadAuto' which is closer to what we normally use for typedef's (albeit normally of structs) Dave > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > +{ > > + rcu_read_unlock(); > > +} > > + > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) > > + > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > + _rcu_read_auto = 'x'; \ > > I'm a bit lost at where _rcu_read_auto is declared. (I could understand > if an earlier macro had created that typedef via concatenating _ with > rcu_read_auto_t, but making the preprocessor drop _t is not possible. Is > this a typo, and if so, why did the compiler not complain?) > > > + rcu_read_lock(); > > + > > #ifdef __cplusplus > > } > > #endif > > > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3226 > Virtualization: qemu.org | libvirt.org > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index 22876d1428..6a25b27d28 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func); }), \ (RCUCBFunc *)g_free); +typedef char rcu_read_auto_t; +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) +{ + rcu_read_unlock(); +} + +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_unlock) + +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ + _rcu_read_auto = 'x'; \ + rcu_read_lock(); + #ifdef __cplusplus } #endif