Message ID | 20241225-event-v1-3-a58c8d63eb70@daynix.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | Improve futex usage | expand |
This fix makes sense. I've also successfully tested with this new "pure" pthread-based QemuEvent in the apple-gfx code where we initially considered using QemuEvent and ran into the destruction issue on macOS. Tested-by: Phil Dennis-Jordan <phil@philjordan.eu> Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu> (Unrelated: email to the devel@daynix.com address in the cc list seems to bounce.) On Wed, 25 Dec 2024 at 06:44, Akihiko Odaki <akihiko.odaki@daynix.com> wrote: > qemu-thread used to abstract pthread primitives into futex for the > QemuEvent implementation of POSIX systems other than Linux. However, > this abstraction has one key difference: unlike futex, pthread > primitives require an explicit destruction, and it must be ordered after > wait and wake operations. > > It would be easier to perform destruction if a wait operation ensures > the corresponding wake operation finishes as POSIX semaphore does, but > that requires to protect state accesses in qemu_event_set() and > qemu_event_wait() with a mutex. On the other hand, real futex does not > need such a protection but needs complex barrier and atomic operations > before wait and wake operations instead. > > Add special implementations of qemu_event_set() and qemu_event_wait() > using pthread primitives. qemu_event_wait() will ensure qemu_event_set() > finishes, and these functions will avoid complex barrier and atomic > operations. > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > --- > util/qemu-thread-posix.c | 45 > ++++++++++++++++++--------------------------- > 1 file changed, 18 insertions(+), 27 deletions(-) > > diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c > index 13459e44c768..805cac444f15 100644 > --- a/util/qemu-thread-posix.c > +++ b/util/qemu-thread-posix.c > @@ -319,28 +319,6 @@ void qemu_sem_wait(QemuSemaphore *sem) > > #ifdef __linux__ > #include "qemu/futex.h" > -#else > -static inline void qemu_futex_wake(QemuEvent *ev, int n) > -{ > - assert(ev->initialized); > - pthread_mutex_lock(&ev->lock); > - if (n == 1) { > - pthread_cond_signal(&ev->cond); > - } else { > - pthread_cond_broadcast(&ev->cond); > - } > - pthread_mutex_unlock(&ev->lock); > -} > - > -static inline void qemu_futex_wait(QemuEvent *ev, unsigned val) > -{ > - assert(ev->initialized); > - pthread_mutex_lock(&ev->lock); > - if (ev->value == val) { > - pthread_cond_wait(&ev->cond, &ev->lock); > - } > - pthread_mutex_unlock(&ev->lock); > -} > #endif > > /* Valid transitions: > @@ -363,7 +341,7 @@ static inline void qemu_futex_wait(QemuEvent *ev, > unsigned val) > > void qemu_event_init(QemuEvent *ev, bool init) > { > -#ifndef __linux__ > +#ifndef CONFIG_LINUX > pthread_mutex_init(&ev->lock, NULL); > pthread_cond_init(&ev->cond, NULL); > #endif > @@ -376,7 +354,7 @@ void qemu_event_destroy(QemuEvent *ev) > { > assert(ev->initialized); > ev->initialized = false; > -#ifndef __linux__ > +#ifndef CONFIG_LINUX > pthread_mutex_destroy(&ev->lock); > pthread_cond_destroy(&ev->cond); > #endif > @@ -386,6 +364,7 @@ void qemu_event_set(QemuEvent *ev) > { > assert(ev->initialized); > > +#ifdef CONFIG_LINUX > /* > * Pairs with both qemu_event_reset() and qemu_event_wait(). > * > @@ -403,6 +382,12 @@ void qemu_event_set(QemuEvent *ev) > qemu_futex_wake_all(ev); > } > } > +#else > + pthread_mutex_lock(&ev->lock); > + qatomic_set(&ev->value, EV_SET); > + pthread_cond_broadcast(&ev->cond); > + pthread_mutex_unlock(&ev->lock); > +#endif > } > > void qemu_event_reset(QemuEvent *ev) > @@ -424,10 +409,9 @@ void qemu_event_reset(QemuEvent *ev) > > void qemu_event_wait(QemuEvent *ev) > { > - unsigned value; > - > assert(ev->initialized); > > +#ifdef CONFIG_LINUX > while (true) { > /* > * qemu_event_wait must synchronize with qemu_event_set even if > it does > @@ -438,7 +422,7 @@ void qemu_event_wait(QemuEvent *ev) > * might miss a qemu_event_set() here but ultimately the memory > barrier > * in qemu_futex_wait() will ensure the check is done correctly. > */ > - value = qatomic_load_acquire(&ev->value); > + unsigned value = qatomic_load_acquire(&ev->value); > if (value == EV_SET) { > break; > } > @@ -467,6 +451,13 @@ void qemu_event_wait(QemuEvent *ev) > */ > qemu_futex_wait(ev, EV_BUSY); > } > +#else > + pthread_mutex_lock(&ev->lock); > + if (qatomic_read(&ev->value) != EV_SET) { > + pthread_cond_wait(&ev->cond, &ev->lock); > + } > + pthread_mutex_unlock(&ev->lock); > +#endif > } > > static __thread NotifierList thread_exit; > > -- > 2.47.1 > >
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index 13459e44c768..805cac444f15 100644 --- a/util/qemu-thread-posix.c +++ b/util/qemu-thread-posix.c @@ -319,28 +319,6 @@ void qemu_sem_wait(QemuSemaphore *sem) #ifdef __linux__ #include "qemu/futex.h" -#else -static inline void qemu_futex_wake(QemuEvent *ev, int n) -{ - assert(ev->initialized); - pthread_mutex_lock(&ev->lock); - if (n == 1) { - pthread_cond_signal(&ev->cond); - } else { - pthread_cond_broadcast(&ev->cond); - } - pthread_mutex_unlock(&ev->lock); -} - -static inline void qemu_futex_wait(QemuEvent *ev, unsigned val) -{ - assert(ev->initialized); - pthread_mutex_lock(&ev->lock); - if (ev->value == val) { - pthread_cond_wait(&ev->cond, &ev->lock); - } - pthread_mutex_unlock(&ev->lock); -} #endif /* Valid transitions: @@ -363,7 +341,7 @@ static inline void qemu_futex_wait(QemuEvent *ev, unsigned val) void qemu_event_init(QemuEvent *ev, bool init) { -#ifndef __linux__ +#ifndef CONFIG_LINUX pthread_mutex_init(&ev->lock, NULL); pthread_cond_init(&ev->cond, NULL); #endif @@ -376,7 +354,7 @@ void qemu_event_destroy(QemuEvent *ev) { assert(ev->initialized); ev->initialized = false; -#ifndef __linux__ +#ifndef CONFIG_LINUX pthread_mutex_destroy(&ev->lock); pthread_cond_destroy(&ev->cond); #endif @@ -386,6 +364,7 @@ void qemu_event_set(QemuEvent *ev) { assert(ev->initialized); +#ifdef CONFIG_LINUX /* * Pairs with both qemu_event_reset() and qemu_event_wait(). * @@ -403,6 +382,12 @@ void qemu_event_set(QemuEvent *ev) qemu_futex_wake_all(ev); } } +#else + pthread_mutex_lock(&ev->lock); + qatomic_set(&ev->value, EV_SET); + pthread_cond_broadcast(&ev->cond); + pthread_mutex_unlock(&ev->lock); +#endif } void qemu_event_reset(QemuEvent *ev) @@ -424,10 +409,9 @@ void qemu_event_reset(QemuEvent *ev) void qemu_event_wait(QemuEvent *ev) { - unsigned value; - assert(ev->initialized); +#ifdef CONFIG_LINUX while (true) { /* * qemu_event_wait must synchronize with qemu_event_set even if it does @@ -438,7 +422,7 @@ void qemu_event_wait(QemuEvent *ev) * might miss a qemu_event_set() here but ultimately the memory barrier * in qemu_futex_wait() will ensure the check is done correctly. */ - value = qatomic_load_acquire(&ev->value); + unsigned value = qatomic_load_acquire(&ev->value); if (value == EV_SET) { break; } @@ -467,6 +451,13 @@ void qemu_event_wait(QemuEvent *ev) */ qemu_futex_wait(ev, EV_BUSY); } +#else + pthread_mutex_lock(&ev->lock); + if (qatomic_read(&ev->value) != EV_SET) { + pthread_cond_wait(&ev->cond, &ev->lock); + } + pthread_mutex_unlock(&ev->lock); +#endif } static __thread NotifierList thread_exit;
qemu-thread used to abstract pthread primitives into futex for the QemuEvent implementation of POSIX systems other than Linux. However, this abstraction has one key difference: unlike futex, pthread primitives require an explicit destruction, and it must be ordered after wait and wake operations. It would be easier to perform destruction if a wait operation ensures the corresponding wake operation finishes as POSIX semaphore does, but that requires to protect state accesses in qemu_event_set() and qemu_event_wait() with a mutex. On the other hand, real futex does not need such a protection but needs complex barrier and atomic operations before wait and wake operations instead. Add special implementations of qemu_event_set() and qemu_event_wait() using pthread primitives. qemu_event_wait() will ensure qemu_event_set() finishes, and these functions will avoid complex barrier and atomic operations. Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> --- util/qemu-thread-posix.c | 45 ++++++++++++++++++--------------------------- 1 file changed, 18 insertions(+), 27 deletions(-)