qemu-thread: fix qemu_event without futexes
This had a possible deadlock that was visible with rcutorture.
qemu_event_set qemu_event_wait
----------------------------------------------------------------
cmpxchg reads FREE, writes BUSY
futex_wait: pthread_mutex_lock
futex_wait: value == BUSY
xchg reads BUSY, writes SET
futex_wake: pthread_cond_broadcast
futex_wait: pthread_cond_wait
<deadlock>
The fix is simply to avoid condvar tricks and do the obvious locking
around pthread_cond_broadcast:
qemu_event_set qemu_event_wait
----------------------------------------------------------------
cmpxchg reads FREE, writes BUSY
futex_wait: pthread_mutex_lock
futex_wait: value == BUSY
xchg reads BUSY, writes SET
futex_wake: pthread_mutex_lock
(blocks)
futex_wait: pthread_cond_wait
(mutex unlocked)
futex_wake: pthread_cond_broadcast
futex_wake: pthread_mutex_unlock
futex_wait: pthread_mutex_unlock
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 41cb23d..50a29d8 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -307,11 +307,13 @@
#else
static inline void futex_wake(QemuEvent *ev, int n)
{
+ 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 futex_wait(QemuEvent *ev, unsigned val)
