diff options
| author | Paolo Bonzini <pbonzini@redhat.com> | 2015-02-02 16:36:51 +0100 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2015-02-02 16:55:09 +0100 |
| commit | 158ef8cbb7e0fe8bb430310924b8bebe5f186e6e (patch) | |
| tree | b8a64778abe8dfcd20d1d43462580d5206b49dd5 /util | |
| parent | 83761b9244ad2ed39d3cfabe8a0e901ab906f7bf (diff) | |
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>
Diffstat (limited to 'util')
| -rw-r--r-- | util/qemu-thread-posix.c | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index 41cb23df0c..50a29d8f7a 100644 --- a/util/qemu-thread-posix.c +++ b/util/qemu-thread-posix.c @@ -307,11 +307,13 @@ 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) |