Handles spurious wake-ups in pthread_join()
Removed 'join_count' from pthread_internal_t and switched to using the flag
PTHREAD_ATTR_FLAG_JOINED to indicate if a thread is being joined. Combined with
a switch to a while loop in pthread_join, this fixes spurious wake-ups but
prevents a thread from being joined multiple times. This is fine for
two reasons:
1) The pthread_join specification allows for undefined behavior when multiple
threads try to join a single thread.
2) There is no thread safe way to allow multiple threads to join a single
thread with the pthread interface. The second thread calling pthread_join
could be pre-empted until the thread is destroyed and its handle reused for
a different thread. Therefore multi-join is always an error.
Bug: https://code.google.com/p/android/issues/detail?id=52255
Change-Id: I8b6784d47620ffdcdbfb14524e7402e21d46c5f7
diff --git a/libc/bionic/pthread_key.cpp b/libc/bionic/pthread_key.cpp
index c793fc6..2ae6519 100644
--- a/libc/bionic/pthread_key.cpp
+++ b/libc/bionic/pthread_key.cpp
@@ -212,16 +212,13 @@
// Clear value in all threads.
pthread_mutex_lock(&gThreadListLock);
for (pthread_internal_t* t = gThreadList; t != NULL; t = t->next) {
- // Avoid zombie threads with a negative 'join_count'. These are really
- // already dead and don't have a TLS area anymore.
-
+ // Skip zombie threads. They don't have a valid TLS area any more.
// Similarly, it is possible to have t->tls == NULL for threads that
// were just recently created through pthread_create() but whose
// startup trampoline (__thread_entry) hasn't been run yet by the
- // scheduler. t->tls will also be NULL after it's stack has been
+ // scheduler. t->tls will also be NULL after a thread's stack has been
// unmapped but before the ongoing pthread_join() is finished.
- // so check for this too.
- if (t->join_count < 0 || !t->tls) {
+ if ((t->attr.flags & PTHREAD_ATTR_FLAG_ZOMBIE) || t->tls == NULL) {
continue;
}