Re: [PATCH] io_uring: complete request via task work in case of DEFER_TASKRUN

[Kanchan Joshi <joshi.k@samsung.com>]

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On Fri, Apr 14, 2023 at 03:53:13PM +0800, Ming Lei wrote:
>So far io_req_complete_post() only covers DEFER_TASKRUN by completing
>request via task work when the request is completed from IOWQ.
>
>However, uring command could be completed from any context, and if io
>uring is setup with DEFER_TASKRUN, the command is required to be
>completed from current context, otherwise wait on IORING_ENTER_GETEVENTS
>can't be wakeup, and may hang forever.
>
>The issue can be observed on removing ublk device, but turns out it is
>one generic issue for uring command & DEFER_TASKRUN, so solve it in
>io_uring core code.

Thanks for sharing, this has been fine for nvme-passthrough side though.
We usually test with DEFER_TASKRUN option, as both fio and t/io_uring
set the option.

>Link: https://lore.kernel.org/linux-block/b3fc9991-4c53-9218-a8cc-5b4dd3952108@kernel.dk/
>Reported-by: Jens Axboe <axboe@kernel.dk>
>Cc: Kanchan Joshi <joshi.k@samsung.com>
>Signed-off-by: Ming Lei <ming.lei@redhat.com>
>---
> io_uring/io_uring.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>index 9083a8466ebf..9f6f92ed60b2 100644
>--- a/io_uring/io_uring.c
>+++ b/io_uring/io_uring.c
>@@ -1012,7 +1012,7 @@ static void __io_req_complete_post(struct io_kiocb *req, unsigned issue_flags)
>
> void io_req_complete_post(struct io_kiocb *req, unsigned issue_flags)
> {
>-	if (req->ctx->task_complete && (issue_flags & IO_URING_F_IOWQ)) {
>+	if (req->ctx->task_complete && req->ctx->submitter_task != current) {
> 		req->io_task_work.func = io_req_task_complete;
> 		io_req_task_work_add(req);

In nvme-side, we always complete in task context, so this seems bit hard
to produce.
But this patch ensures that task-work is setup if it is needed, and
caller/driver did not get to set that explicitly. So looks fine to me.
FWIW, I do not see regression in nvme tests.

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