io-wq: don't retry task_work creation failure on fatal conditions

We don't want to be retrying task_work creation failure if there's
an actual signal pending for the parent task. If we do, then we can
enter an infinite loop of perpetually retrying and each retry failing
with -ERESTARTNOINTR because a signal is pending.

Fixes: 3146cba ("io-wq: make worker creation resilient against signals")
Reported-by: Florian Fischer <florian.fl.fischer@fau.de>
Link: https://lore.kernel.org/io-uring/20211202165606.mqryio4yzubl7ms5@pasture/
Tested-by: Florian Fischer <florian.fl.fischer@fau.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -714,6 +714,13 @@ static bool io_wq_work_match_all(struct io_wq_work *work, void *data)
 
 static inline bool io_should_retry_thread(long err)
 {
+	/*
+	 * Prevent perpetual task_work retry, if the task (or its group) is
+	 * exiting.
+	 */
+	if (fatal_signal_pending(current))
+		return false;
+
 	switch (err) {
 	case -EAGAIN:
 	case -ERESTARTSYS: