RE: [PATCH] io_uring: simplify the SQPOLL thread check when cancelling requests

lizetao
2025-01-12T15:45:45Z

Hi,

> -----Original Message-----
> From: Bui Quang Minh <minhquangbui99@gmail.com>
> Sent: Sunday, January 12, 2025 10:34 PM
> To: linux-kernel@vger.kernel.org
> Cc: Bui Quang Minh <minhquangbui99@gmail.com>; Jens Axboe
> <axboe@kernel.dk>; Pavel Begunkov <asml.silence@gmail.com>; io-
> uring@vger.kernel.org;
> syzbot+3c750be01dab672c513d@syzkaller.appspotmail.com; lizetao
> <lizetao1@huawei.com>
> Subject: [PATCH] io_uring: simplify the SQPOLL thread check when cancelling
> requests
>
> In io_uring_try_cancel_requests, we check whether sq_data->thread ==
> current to determine if the function is called by the SQPOLL thread to do iopoll
> when IORING_SETUP_SQPOLL is set. This check can race with the SQPOLL
> thread termination.
>
> io_uring_cancel_generic is used in 2 places: io_uring_cancel_generic and
> io_ring_exit_work. In io_uring_cancel_generic, we have the information
> whether the current is SQPOLL thread already. In io_ring_exit_work, in case
> the SQPOLL thread reaches this path, we don't need to iopoll and leave that for
> io_uring_cancel_generic to handle.
>
> So to avoid the racy check, this commit adds a boolean flag to
> io_uring_try_cancel_requests to determine if we need to do iopoll inside the
> function and only sets this flag in io_uring_cancel_generic when the current is
> SQPOLL thread.
>
> Reported-by: syzbot+3c750be01dab672c513d@syzkaller.appspotmail.com
> Reported-by: Li Zetao <lizetao1@huawei.com>
> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> ---
> io_uring/io_uring.c | 21 +++++++++++++++------
> 1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index
> ff691f37462c..f28ea1254143 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -143,7 +143,8 @@ struct io_defer_entry {
>
> static bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> struct io_uring_task *tctx,
> - bool cancel_all);
> + bool cancel_all,
> + bool force_iopoll);
>
> static void io_queue_sqe(struct io_kiocb *req);
>
> @@ -2898,7 +2899,12 @@ static __cold void io_ring_exit_work(struct
> work_struct *work)
> if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
> io_move_task_work_from_local(ctx);
>
> - while (io_uring_try_cancel_requests(ctx, NULL, true))
> + /*
> + * Even if SQPOLL thread reaches this path, don't force
> + * iopoll here, let the io_uring_cancel_generic handle
> + * it.

Just curious, will sq_thread enter this io_ring_exit_work path?

> + */
> + while (io_uring_try_cancel_requests(ctx, NULL, true, false))
> cond_resched();
>
> if (ctx->sq_data) {
> @@ -3066,7 +3072,8 @@ static __cold bool io_uring_try_cancel_iowq(struct
> io_ring_ctx *ctx)
>
> static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
> struct io_uring_task *tctx,
> - bool cancel_all)
> + bool cancel_all,
> + bool force_iopoll)
> {
> struct io_task_cancel cancel = { .tctx = tctx, .all = cancel_all, };
> enum io_wq_cancel cret;
> @@ -3096,7 +3103,7 @@ static __cold bool
> io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
>
> /* SQPOLL thread does its own polling */
> if ((!(ctx->flags & IORING_SETUP_SQPOLL) && cancel_all) ||
> - (ctx->sq_data && ctx->sq_data->thread == current)) {
> + force_iopoll) {
> while (!wq_list_empty(&ctx->iopoll_list)) {
> io_iopoll_try_reap_events(ctx);
> ret = true;
> @@ -3169,13 +3176,15 @@ __cold void io_uring_cancel_generic(bool
> cancel_all, struct io_sq_data *sqd)
> continue;
> loop |= io_uring_try_cancel_requests(node-
> >ctx,
> current->io_uring,
> - cancel_all);
> + cancel_all,
> + false);
> }
> } else {
> list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
> loop |= io_uring_try_cancel_requests(ctx,
> current-
> >io_uring,
> - cancel_all);
> + cancel_all,
> + true);
> }
>
> if (loop) {
> --
> 2.43.0
>

Maybe you miss something, just like Begunkov mentioned in your last version patch:

io_uring_cancel_generic
WARN_ON_ONCE(sqd && sqd->thread != current);

This WARN_ON_ONCE will never be triggered, so you could remove it.

---
Li Zetao