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qemu-thread: do not use PTHREAD_MUTEX_ERRORCHECK 

PTHREAD_MUTEX_ERRORCHECK is completely broken with respect to fork.
The way to safely do fork is to bring all threads to a quiescent
state by acquiring locks (either in callers---as we do for the
iothread mutex---or using pthread_atfork's prepare callbacks)
and then release them in the child.

The problem is that releasing error-checking locks in the child
fails under glibc with EPERM, because the mutex stores a different
owner tid than the duplicated thread in the child process.  We
could make it work for locks acquired via pthread_atfork, by
recreating the mutex in the child instead of unlocking it
(we know that there are no other threads that could have taken
the mutex; but when the lock is acquired in fork's caller
that would not be possible.

The simplest solution is just to forgo error checking.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2015-03-05 16:47:14 +01:00
parent 811c572776
commit 24fa90499f
1 changed files with 1 additions and 5 deletions
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6
util/qemu-thread-posix.c
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@ -51,12 +51,8 @@ static void error_exit(int err, const char *msg)
void qemu_mutex_init(QemuMutex *mutex)
{
int err;
pthread_mutexattr_t mutexattr;
pthread_mutexattr_init(&mutexattr);
pthread_mutexattr_settype(&mutexattr, PTHREAD_MUTEX_ERRORCHECK);
err = pthread_mutex_init(&mutex->lock, &mutexattr);
pthread_mutexattr_destroy(&mutexattr);
err = pthread_mutex_init(&mutex->lock, NULL);
if (err)
error_exit(err, __func__);
}