[PATCH v2] io_uring: check for presence of task_work rather than TIF_NOTIFY_SIGNAL

[Jens Axboe <axboe@kernel.dk>]

If some part of the kernel adds task_work that needs executing, in terms
of signaling it'll generally use TWA_SIGNAL or TWA_RESUME. Those two
directly translate to TIF_NOTIFY_SIGNAL or TIF_NOTIFY_RESUME, and can
be used for a variety of use case outside of task_work.

However, io_cqring_wait_schedule() only tests explicitly for
TIF_NOTIFY_SIGNAL. This means it can miss if task_work got added for
the task, but used a different kind of signaling mechanism (or none at
all). Normally this doesn't matter as any task_work will be run once
the task exits to userspace, except if:

1) The ring is setup with DEFER_TASKRUN
2) The local work item may generate normal task_work

For condition 2, this can happen when closing a file and it's the final
put of that file, for example. This can cause stalls where a task is
waiting to make progress inside io_cqring_wait(), but there's nothing else
that will wake it up. Hence change the "should we schedule or loop around"
check to check for the presence of task_work explicitly, rather than just
TIF_NOTIFY_SIGNAL as the mechanism. While in there, also change the
ordering of what type of task_work first in terms of ordering, to both
make it consistent with other task_work runs in io_uring, but also to
better handle the case of defer task_work generating normal task_work,
like in the above example.

Reported-by: Jan Hendrik Farr <kernel@jfarr.cc>
Link: https://github.com/axboe/liburing/issues/1235
Cc: stable@vger.kernel.org
Fixes: 846072f16eed ("io_uring: mimimise io_cqring_wait_schedule")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

V2:
	- Improve commit message
	- In addition to re-ordering the task_work type runs, change
	  the io_cqring_wait_schedule() check to explicitly check
	  for the presence of task_work rather than the notification
	  method.

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 1aca501efaf6..11c455b638a2 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2461,7 +2461,7 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 		return 1;
 	if (unlikely(!llist_empty(&ctx->work_llist)))
 		return 1;
-	if (unlikely(test_thread_flag(TIF_NOTIFY_SIGNAL)))
+	if (unlikely(task_work_pending(current)))
 		return 1;
 	if (unlikely(task_sigpending(current)))
 		return -EINTR;
@@ -2568,9 +2568,9 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, u32 flags,
 		 * If we got woken because of task_work being processed, run it
 		 * now rather than let the caller do another wait loop.
 		 */
-		io_run_task_work();
 		if (!llist_empty(&ctx->work_llist))
 			io_run_local_work(ctx, nr_wait);
+		io_run_task_work();
 
 		/*
 		 * Non-local task_work will be run on exit to userspace, but

-- 
Jens Axboe