On 1/12/25 22:45, lizetao wrote:
Hi,-----Original Message----- From: Bui Quang Minh <minhquangbui99@xxxxxxxxx> Sent: Sunday, January 12, 2025 10:34 PM To: linux-kernel@xxxxxxxxxxxxxxx Cc: Bui Quang Minh <minhquangbui99@xxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>; Pavel Begunkov <asml.silence@xxxxxxxxx>; io- uring@xxxxxxxxxxxxxxx; syzbot+3c750be01dab672c513d@xxxxxxxxxxxxxxxxxxxxxxxxx; lizetao <lizetao1@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx Reported-by: Li Zetao <lizetao1@xxxxxxxxxx> Signed-off-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx> --- 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?
AFAIK, yes. The SQPOLL thread is created with create_io_thread, this function creates a new task with CLONE_FILES. So all the open files is shared. There will be case that the parent closes its io_uring file and SQPOLL thread become the only owner of that file. So it can reach this path when terminating.
+ */ + 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) { -- 2.43.0Maybe 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.
He meant that we don't need to annotate sqd->thread access in this debug check. The io_uring_cancel_generic function has assumption that the sgd is not NULL only when it's called by a SQPOLL thread. So the check means to ensure this assumption. A data race happens only when this function is called by other tasks than the SQPOLL thread, so it can race with the SQPOLL termination. However, the sgd is not NULL only when this function is called by SQPOLL thread. In normal situation following the io_uring_cancel_generic's assumption, the data race cannot happen. And in case the assumption is broken, the warning almost always is triggered even if data race happens. So we can ignore the race here.
Thanks, Quang Minh.
