On Sun, May 11, 2025 at 03:08:18PM +0900, Akihiko Odaki wrote: > futex(2) - Linux manual page > https://man7.org/linux/man-pages/man2/futex.2.html > > Note that a wake-up can also be caused by common futex usage patterns > > in unrelated code that happened to have previously used the futex > > word's memory location (e.g., typical futex-based implementations of > > Pthreads mutexes can cause this under some conditions). Therefore, > > callers should always conservatively assume that a return value of 0 > > can mean a spurious wake-up, and use the futex word's value (i.e., > > the user-space synchronization scheme) to decide whether to continue > > to block or not.
I'm just curious - do you know when this will happen? AFAIU, QEMU uses futex always on private mappings, internally futex does use (mm, HVA) tuple to index a futex, afaict. Hence, I don't see how it can get spurious wakeups.. And _if_ it happens, since mm pointer can't change it must mean the HVA of the futex word is reused, it sounds like an UAF user bug to me instead. I checked the man-pages git repo, this line was introduced in: https://github.com/mkerrisk/man-pages/commit/4b35dc5dabcf356ce6dcb1f949f7b00e76c7587d I also didn't see details yet in commit message on why that paragraph was added. And.. > > Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com> > --- > include/qemu/futex.h | 9 +++++++++ > tests/unit/test-aio-multithread.c | 4 +++- > util/qemu-thread-posix.c | 28 ++++++++++++++++------------ > 3 files changed, 28 insertions(+), 13 deletions(-) > > diff --git a/include/qemu/futex.h b/include/qemu/futex.h > index 91ae88966e12..f57774005330 100644 > --- a/include/qemu/futex.h > +++ b/include/qemu/futex.h > @@ -24,6 +24,15 @@ static inline void qemu_futex_wake(void *f, int n) > qemu_futex(f, FUTEX_WAKE, n, NULL, NULL, 0); > } > > +/* > + * Note that a wake-up can also be caused by common futex usage patterns in > + * unrelated code that happened to have previously used the futex word's > + * memory location (e.g., typical futex-based implementations of Pthreads > + * mutexes can cause this under some conditions). Therefore, callers should .. another thing that was unclear to me is, here it's mentioning "typical futex-based implementations of pthreads mutexes..", but here qemu_futex_wait() is using raw futex without any pthread impl. Does it also mean that this may not be applicable to whatever might cause a spurious wakeup? > + * always conservatively assume that it is a spurious wake-up, and use the > futex > + * word's value (i.e., the user-space synchronization scheme) to decide > whether > + * to continue to block or not. > + */ > static inline void qemu_futex_wait(void *f, unsigned val) > { > while (qemu_futex(f, FUTEX_WAIT, (int) val, NULL, NULL, 0)) { > diff --git a/tests/unit/test-aio-multithread.c > b/tests/unit/test-aio-multithread.c > index 08d4570ccb14..8c2e41545a29 100644 > --- a/tests/unit/test-aio-multithread.c > +++ b/tests/unit/test-aio-multithread.c > @@ -305,7 +305,9 @@ static void mcs_mutex_lock(void) > prev = qatomic_xchg(&mutex_head, id); > if (prev != -1) { > qatomic_set(&nodes[prev].next, id); > - qemu_futex_wait(&nodes[id].locked, 1); > + while (qatomic_read(&nodes[id].locked) == 1) { > + qemu_futex_wait(&nodes[id].locked, 1); > + } > } > } > > diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c > index b2e26e21205b..eade5311d175 100644 > --- a/util/qemu-thread-posix.c > +++ b/util/qemu-thread-posix.c > @@ -428,17 +428,21 @@ void qemu_event_wait(QemuEvent *ev) > > assert(ev->initialized); > > - /* > - * qemu_event_wait must synchronize with qemu_event_set even if it does > - * not go down the slow path, so this load-acquire is needed that > - * synchronizes with the first memory barrier in qemu_event_set(). > - * > - * If we do go down the slow path, there is no requirement at all: we > - * 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); > - if (value != EV_SET) { > + while (true) { > + /* > + * qemu_event_wait must synchronize with qemu_event_set even if it > does > + * not go down the slow path, so this load-acquire is needed that > + * synchronizes with the first memory barrier in qemu_event_set(). > + * > + * If we do go down the slow path, there is no requirement at all: we > + * 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); > + if (value == EV_SET) { > + break; > + } > + > if (value == EV_FREE) { > /* > * Leave the event reset and tell qemu_event_set that there are > @@ -452,7 +456,7 @@ void qemu_event_wait(QemuEvent *ev) > * like the load above. > */ > if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) { > - return; > + break; > } > } > > > -- > 2.49.0 > -- Peter Xu
