binder: use wake_up_pollfree()

wake_up_poll() uses nr_exclusive=1, so it's not guaranteed to wake up
all exclusive waiters.  Yet, POLLFREE *must* wake up all waiters.  epoll
and aio poll are fortunately not affected by this, but it's very
fragile.  Thus, the new function wake_up_pollfree() has been introduced.

Convert binder to use wake_up_pollfree().
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: f5cb779b ("ANDROID: binder: remove waitqueue when thread exits.")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211209010455.42744-3-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>

drivers/android/binder.c

@@ -4422,23 +4422,20 @@ static int binder_thread_release(struct binder_proc *proc,
 	__release(&t->lock);
 
 	/*
-	 * If this thread used poll, make sure we remove the waitqueue
-	 * from any epoll data structures holding it with POLLFREE.
-	 * waitqueue_active() is safe to use here because we're holding
-	 * the inner lock.
+	 * If this thread used poll, make sure we remove the waitqueue from any
+	 * poll data structures holding it.
 	 */
-	if ((thread->looper & BINDER_LOOPER_STATE_POLL) &&
-	    waitqueue_active(&thread->wait)) {
-		wake_up_poll(&thread->wait, EPOLLHUP | POLLFREE);
-	}
+	if (thread->looper & BINDER_LOOPER_STATE_POLL)
+		wake_up_pollfree(&thread->wait);
 
 	binder_inner_proc_unlock(thread->proc);
 
 	/*
-	 * This is needed to avoid races between wake_up_poll() above and
-	 * and ep_remove_waitqueue() called for other reasons (eg the epoll file
-	 * descriptor being closed); ep_remove_waitqueue() holds an RCU read
-	 * lock, so we can be sure it's done after calling synchronize_rcu().
+	 * This is needed to avoid races between wake_up_pollfree() above and
+	 * someone else removing the last entry from the queue for other reasons
+	 * (e.g. ep_remove_wait_queue() being called due to an epoll file
+	 * descriptor being closed).  Such other users hold an RCU read lock, so
+	 * we can be sure they're done after we call synchronize_rcu().
 	 */
 	if (thread->looper & BINDER_LOOPER_STATE_POLL)
 		synchronize_rcu();