Minimize the potential for deadlocks between an exiting thread and it's

joiner by making sure all locks and unlocks occur in the same order. For
the record the lock order is: DEAD_LIST, THREAD_LIST, exiting thread, joiner
thread.

Approved by: re/rwatson

1 files changed, 18 insertions, 2 deletions

diff --git a/lib/libthr/thread/thr_exit.c b/lib/libthr/thread/thr_exit.c
index 8c4a060..dd510b1 100644
--- a/lib/libthr/thread/thr_exit.c
+++ b/lib/libthr/thread/thr_exit.c
@@ -94,7 +94,7 @@ _thread_exit_cleanup(void)
 void
 _pthread_exit(void *status)
 {
-	pthread_t pthread;
+	pthread_t pthread, joiner;
 	int exitNow = 0;
 
 	/* Check if this thread is already in the process of exiting: */
@@ -122,10 +122,27 @@ _pthread_exit(void *status)
 		_thread_cleanupspecific();
 	}
 
+retry:
+	/*
+	 * Proper lock order, to minimize deadlocks, between joining
+	 * and exiting threads is: DEAD_LIST, THREAD_LIST, exiting, joiner.
+	 * In order to do this *and* protect from races, we must resort
+	 * this test-and-retry loop.
+	 */
+	joiner = curthread->joiner;
+
 	/* Lock the dead list first to maintain correct lock order */
 	DEAD_LIST_LOCK;
+	THREAD_LIST_LOCK;
 	_thread_critical_enter(curthread);
 
+	if (joiner != curthread->joiner) {
+		_thread_critical_exit(curthread);
+		THREAD_LIST_UNLOCK;
+		DEAD_LIST_UNLOCK;
+		goto retry;
+	}
+
 	/* Check if there is a thread joining this one: */
 	if (curthread->joiner != NULL) {
 		pthread = curthread->joiner;
@@ -151,7 +168,6 @@ _pthread_exit(void *status)
 	 * Add this thread to the list of dead threads, and
 	 * also remove it from the active threads list.
 	 */
-	THREAD_LIST_LOCK;
 	TAILQ_INSERT_HEAD(&_dead_list, curthread, dle);
 	TAILQ_REMOVE(&_thread_list, curthread, tle);
 	PTHREAD_SET_STATE(curthread, PS_DEAD);