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futex: Check value after qemu_futex_wait()

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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.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250529-event-v5-1-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Akihiko Odaki 2025-05-29 14:45:50 +09:00 • committed by Paolo Bonzini
parent 4cdc489eb9
commit 6e2d11bf04
3 changed files with 24 additions and 13 deletions
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9
include/qemu/futex.h
View file

@ -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
* 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)) {

4
tests/unit/test-aio-multithread.c
View file

@ -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);
}
}
}

24
util/qemu-thread-posix.c
View file

@ -428,17 +428,17 @@ 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().
*/
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 +452,7 @@ void qemu_event_wait(QemuEvent *ev)
* like the load above.
*/
if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
return;
break;
}
}