From: Paolo Bonzini <pbonz...@redhat.com> 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-sta...@nongnu.org Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> (cherry picked from commit 158ef8cbb7e0fe8bb430310924b8bebe5f186e6e) Signed-off-by: Michael Roth <mdr...@linux.vnet.ibm.com> --- util/qemu-thread-posix.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index d05a649..bb14ad4 100644 --- a/util/qemu-thread-posix.c +++ b/util/qemu-thread-posix.c @@ -306,11 +306,13 @@ static inline void futex_wait(QemuEvent *ev, unsigned val) #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) -- 1.9.1