[PATCH v2] io_uring: clear TIF_NOTIFY_SIGNAL if set and task_work not available

[PATCH v2] io_uring: clear TIF_NOTIFY_SIGNAL if set and task_work not available

[Jens Axboe]

If we have io-wq or SQPOLL setting the task_work notify signal but the
task itself doesn't have task_work to process, we don't clear the
flag and hence will enter a repeated check loop if we're waiting on
events or file/buf references to go away.

This was introduced in a recent patch which eliminated gating the
task_work run on just that flag, but that fix meant that we know don't
clear the flag if the task itsel doesn't have task_work to run.

Cc: stable@vger.kernel.org
Fixes: 46a525e199e4 ("io_uring: don't gate task_work run on TIF_NOTIFY_SIGNAL")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

V2
	- Add comment
	- Keep the clear-if-set logic, just move it outside the tw check

diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index cef5ff924e63..a70f8ec88bf3 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -238,9 +238,16 @@ static inline unsigned int io_sqring_entries(struct io_ring_ctx *ctx)
 
 static inline int io_run_task_work(void)
 {
+	/*
+	 * Always check-and-clear the task_work notification signal. The
+	 * current task may have it set if io-wq or SQPOLL sets it, but not
+	 * have any task_work itself. This can prevent the current task from
+	 * waiting on events efficiently, as interruptible sleeps will turn
+	 * into busy loops.
+	 */
+	if (test_thread_flag(TIF_NOTIFY_SIGNAL))
+		clear_notify_signal();
 	if (task_work_pending(current)) {
-		if (test_thread_flag(TIF_NOTIFY_SIGNAL))
-			clear_notify_signal();
 		__set_current_state(TASK_RUNNING);
 		task_work_run();
 		return 1;

-- 
Jens Axboe