From f36826da252195aa5d378f4a53351b191fc6d531 Mon Sep 17 00:00:00 2001 From: mtm Date: Tue, 27 May 2003 21:48:42 +0000 Subject: 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 --- lib/libthr/thread/thr_exit.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) (limited to 'lib/libthr') 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); -- cgit v1.1