io_uring: don't gate task_work run on TIF_NOTIFY_SIGNAL

commit 46a525e199 upstream.

This isn't a reliable mechanism to tell if we have task_work pending, we
really should be looking at whether we have any items queued. This is
problematic if forward progress is gated on running said task_work. One
such example is reading from a pipe, where the write side has been closed
right before the read is started. The fput() of the file queues TWA_RESUME
task_work, and we need that task_work to be run before ->release() is
called for the pipe. If ->release() isn't called, then the read will sit
forever waiting on data that will never arise.

Fix this by io_run_task_work() so it checks if we have task_work pending
rather than rely on TIF_NOTIFY_SIGNAL for that. The latter obviously
doesn't work for task_work that is queued without TWA_SIGNAL.

Reported-by: Christiano Haesbaert <haesbaert@haesbaert.org>
Cc: stable@vger.kernel.org
Link: https://github.com/axboe/liburing/issues/665
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

io_uring/io-wq.c

@@ -513,7 +513,7 @@ static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
 
 static bool io_flush_signals(void)
 {
-	if (unlikely(test_thread_flag(TIF_NOTIFY_SIGNAL))) {
+	if (test_thread_flag(TIF_NOTIFY_SIGNAL) || current->task_works) {
 		__set_current_state(TASK_RUNNING);
 		tracehook_notify_signal();
 		return true;