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>
---
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
+ * 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;
}
}
--
2.47.1This is somewhat orthogonal to the issue being addressed here, but:
While reading the man page to make sense of this patch, I noticed the
following:
> If the futex value does not match val, then the call fails
> immediately with the error EAGAIN.
And qemu_futex_wait does not seem to handle that case. In fact it seems
like it would take the default: abort(); code path?
If I've got this right, I'm surprised there aren't spurious abort()s
happening, but I suppose QemuEvent and qemu_futex_* are used fairly
sparingly and in low-contention areas.
On Wed, 25 Dec 2024 at 06:44, Akihiko Odaki <akihiko.odaki@daynix.com>
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.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>
In any case, for the specific issue addressed here:
Reviewed-by: Phil Dennis-Jordan <phil@philjordan.eu>
> ---
> 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
> + * 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;
> }
> }
>
>
> --
> 2.47.1
>
>
On 2024/12/28 20:11, Phil Dennis-Jordan wrote: > This is somewhat orthogonal to the issue being addressed here, but: > While reading the man page to make sense of this patch, I noticed the > following: > > > If the futex value does not match val, then the call fails > > immediately with the error EAGAIN. > > And qemu_futex_wait does not seem to handle that case. In fact it seems > like it would take the default: abort(); code path? It's handled as EWOULDBLOCK. The man page says: > Note: on Linux, the symbolic names EAGAIN and EWOULDBLOCK (both of > which appear in different parts of the kernel futex code) have the > same value. > > If I've got this right, I'm surprised there aren't spurious abort()s > happening, but I suppose QemuEvent and qemu_futex_* are used fairly > sparingly and in low-contention areas. QemuLockCnt, which relies on qemu_futex_*, is used in more contended areas so it will cause trouble if qemu_futex_* is broken.
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