[PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running

[Jens Axboe <axboe@kernel.dk>]

An earlier commit:

b7db41c9e03b ("io_uring: fix regression with always ignoring signals in io_cqring_wait()")

ensured that we didn't get stuck waiting for eventfd reads when it's
registered with the io_uring ring for event notification, but we still
have a gap where the task can be waiting on other events in the kernel
and need a bigger nudge to make forward progress.

Ensure that we use signaled notifications for a task that isn't currently
running, to be certain the work is seen and processed immediately.

Cc: stable@vger.kernel.org # v5.7+
Reported-by: Josef <josef.grieb@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index e9b27cdaa735..443eecdfeda9 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1712,21 +1712,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
 	struct io_ring_ctx *ctx = req->ctx;
 	int ret, notify = TWA_RESUME;
 
+	ret = __task_work_add(tsk, cb);
+	if (unlikely(ret))
+		return ret;
+
 	/*
 	 * SQPOLL kernel thread doesn't need notification, just a wakeup.
-	 * If we're not using an eventfd, then TWA_RESUME is always fine,
-	 * as we won't have dependencies between request completions for
-	 * other kernel wait conditions.
+	 * For any other work, use signaled wakeups if the task isn't
+	 * running to avoid dependencies between tasks or threads. If
+	 * the issuing task is currently waiting in the kernel on a thread,
+	 * and same thread is waiting for a completion event, then we need
+	 * to ensure that the issuing task processes task_work. TWA_SIGNAL
+	 * is needed for that.
 	 */
 	if (ctx->flags & IORING_SETUP_SQPOLL)
 		notify = 0;
-	else if (ctx->cq_ev_fd)
+	else if (READ_ONCE(tsk->state) != TASK_RUNNING)
 		notify = TWA_SIGNAL;
 
-	ret = task_work_add(tsk, cb, notify);
-	if (!ret)
-		wake_up_process(tsk);
-	return ret;
+	__task_work_notify(tsk, notify);
+	wake_up_process(tsk);
+	return 0;
 }
 
 static void __io_req_task_cancel(struct io_kiocb *req, int error)
-- 
2.28.0