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_join.cpp b/libc/bionic/pthread_join.cpp
index e6acc34..7e022c2 100644
--- a/libc/bionic/pthread_join.cpp
+++ b/libc/bionic/pthread_join.cpp
@@ -30,7 +30,7 @@
#include "pthread_accessor.h"
-int pthread_join(pthread_t t, void ** ret_val) {
+int pthread_join(pthread_t t, void** ret_val) {
if (t == pthread_self()) {
return EDEADLK;
}
@@ -44,25 +44,19 @@
return EINVAL;
}
- // Wait for thread death when needed.
+ if (thread->attr.flags & PTHREAD_ATTR_FLAG_JOINED) {
+ return EINVAL;
+ }
- // If the 'join_count' is negative, this is a 'zombie' thread that
- // is already dead and without stack/TLS. Otherwise, we need to increment 'join-count'
- // and wait to be signaled
- int count = thread->join_count;
- if (count >= 0) {
- thread->join_count += 1;
+ // Signal our intention to join, and wait for the thread to exit.
+ thread->attr.flags |= PTHREAD_ATTR_FLAG_JOINED;
+ while ((thread->attr.flags & PTHREAD_ATTR_FLAG_ZOMBIE) == 0) {
pthread_cond_wait(&thread->join_cond, &gThreadListLock);
- count = --thread->join_count;
}
if (ret_val) {
*ret_val = thread->return_value;
}
- // Remove thread from thread list when we're the last joiner or when the
- // thread was already a zombie.
- if (count <= 0) {
- _pthread_internal_remove_locked(thread.get());
- }
+ _pthread_internal_remove_locked(thread.get());
return 0;
}