[Qemu-devel] [PATCH 1/2] qemu-thread: do not use PTHREAD_MUTEX_ERRORCHECK

[Paolo Bonzini <pbonzini@redhat.com>]

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>
---
 util/qemu-thread-posix.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 50a29d8..ba67cec 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -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__);
 }
-- 
2.3.0