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qemu-thread: fix qemu_event without futexes

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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>
This commit is contained in:
Paolo Bonzini 2015-02-02 16:36:51 +01:00
parent 83761b9244
commit 158ef8cbb7
1 changed files with 2 additions and 0 deletions
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2
util/qemu-thread-posix.c
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@ -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)