From: Paolo Bonzini <pbonzini@redhat.com>
To: qemu-devel@nongnu.org
Cc: peter.maydell@linaro.org, qemu-stable@nongnu.org
Subject: [Qemu-devel] [PATCH] qemu-thread: fix qemu_event without futexes
Date: Mon, 2 Feb 2015 16:50:00 +0100 [thread overview]
Message-ID: <1422892200-11817-1-git-send-email-pbonzini@redhat.com> (raw)
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>
---
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 41cb23d..50a29d8 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)
--
1.8.3.1
reply other threads:[~2015-02-02 15:50 UTC|newest]
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