fix race condition in main_notify.c

The function wait_main_task() is racy: if the function wakeup_via_cond() sees the mutex is locked just before wait_main_task() executes pthread_cond_wait(), the main thread blocks forever. Work around this issue by adding a "pending" flag just like in my notify.c code. A standards-compliant solution should be implemented later. git-svn-id: https://svn.musicpd.org/mpd/trunk@7365 09075e82-0dd4-0310-85a5-a0d7c8717e4f

fix race condition in main_notify.c
3c3620b3 · Max Kellermann · Eric Wong · 7c952c4f · 3c3620b3
Commit 3c3620b3 authored Apr 17, 2008 by Max Kellermann Committed by Eric Wong Apr 17, 2008
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Showing with 6 additions and 1 deletion

main_notify.c src/main_notify.c +6 -1

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--- a/src/main_notify.c
+++ b/src/main_notify.c
@@ -30,6 +30,7 @@ static int main_pipe[2];
 static pthread_t main_task;
 static pthread_cond_t main_wakeup = PTHREAD_COND_INITIALIZER;
 static pthread_mutex_t main_wakeup_mutex = PTHREAD_MUTEX_INITIALIZER;
+static volatile int pending;
 static pthread_mutex_t select_mutex = PTHREAD_MUTEX_INITIALIZER;

 static int ioops_fdset(fd_set * rfds,
@@ -94,6 +95,8 @@ void wakeup_main_task(void)
 {
 	assert(!pthread_equal(main_task, pthread_self()));

+	pending = 1;
+
 	if (!wakeup_via_pipe())
 		pthread_cond_signal(&main_wakeup);
 }
@@ -115,7 +118,9 @@ void wait_main_task(void)
 	assert(pthread_equal(main_task, pthread_self()));

 	pthread_mutex_lock(&main_wakeup_mutex);
-	pthread_cond_wait(&main_wakeup, &main_wakeup_mutex);
+	if (!pending)
+		pthread_cond_wait(&main_wakeup, &main_wakeup_mutex);
+	pending = 0;
 	pthread_mutex_unlock(&main_wakeup_mutex);
 }