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
to ensure ordering between the two functions.
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 to ensure ordering between them.
Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>
Tested-by: Phil Dennis-Jordan <p...@philjordan.eu>
Reviewed-by: Phil Dennis-Jordan <p...@philjordan.eu>
---
util/qemu-thread-posix.c | 47 +++++++++++++++++++++--------------------------
1 file changed, 21 insertions(+), 26 deletions(-)
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index dfe26b8d020c..6dc6f2b25dce 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -319,28 +319,6 @@ void qemu_sem_wait(QemuSemaphore *sem)
#ifdef CONFIG_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:
@@ -387,12 +365,17 @@ void qemu_event_set(QemuEvent *ev)
assert(ev->initialized);
/*
- * Pairs with both qemu_event_reset() and qemu_event_wait().
+ * Pairs with qemu_event_wait() (on Linux) and qemu_event_reset().
*
* qemu_event_set has release semantics, but because it *loads*
* ev->value we need a full memory barrier here.
+ *
+ * qemu_event_wait() do not have a paired barrier on non-Linux because
+ * ev->lock ensures ordering.
*/
smp_mb();
+
+#ifdef CONFIG_LINUX
if (qatomic_read(&ev->value) != EV_SET) {
int old = qatomic_xchg(&ev->value, EV_SET);
@@ -403,6 +386,12 @@ 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 +413,9 @@ 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 +426,7 @@ 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 +455,13 @@ void qemu_event_wait(QemuEvent *ev)
*/
qemu_futex_wait(ev, EV_BUSY);
}
+#else
+ pthread_mutex_lock(&ev->lock);
+ while (qatomic_read(&ev->value) != EV_SET) {
+ pthread_cond_wait(&ev->cond, &ev->lock);
+ }
+ pthread_mutex_unlock(&ev->lock);
+#endif
}
static __thread NotifierList thread_exit;
--
2.49.0
