io_uring: move local task_work in exit cancel loop

With IORING_SETUP_DEFER_TASKRUN, task work is queued to ctx->work_llist
(local work) rather than the fallback list. During io_ring_exit_work(),
io_move_task_work_from_local() was called once before the cancel loop,
moving work from work_llist to fallback_llist.

However, task work can be added to work_llist during the cancel loop
itself. There are two cases:

1) io_kill_timeouts() is called from io_uring_try_cancel_requests() to
cancel pending timeouts, and it adds task work via io_req_queue_tw_complete()
for each cancelled timeout:

2) URING_CMD requests like ublk can be completed via
io_uring_cmd_complete_in_task() from ublk_queue_rq() during canceling,
given ublk request queue is only quiesced when canceling the 1st uring_cmd.

Since io_allowed_defer_tw_run() returns false in io_ring_exit_work()
(kworker != submitter_task), io_run_local_work() is never invoked,
and the work_llist entries are never processed. This causes
io_uring_try_cancel_requests() to loop indefinitely, resulting in
100% CPU usage in kworker threads.

Fix this by moving io_move_task_work_from_local() inside the cancel
loop, ensuring any work on work_llist is moved to fallback before
each cancel attempt.

Cc: stable@vger.kernel.org
Fixes: c0e0d6ba25f1 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 87a87396e9409f1bb1e53206cba0d4b914745e52..b7a077c11c21a99be2a462e2bfc2eb114c43152c 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3003,12 +3003,12 @@ static __cold void io_ring_exit_work(struct work_struct *work)
                        mutex_unlock(&ctx->uring_lock);
                }
 
-               if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
-                       io_move_task_work_from_local(ctx);
-
                /* The SQPOLL thread never reaches this path */
-               while (io_uring_try_cancel_requests(ctx, NULL, true, false))
+               do {
+                       if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
+                               io_move_task_work_from_local(ctx);
                        cond_resched();
+               } while (io_uring_try_cancel_requests(ctx, NULL, true, false));
 
                if (ctx->sq_data) {
                        struct io_sq_data *sqd = ctx->sq_data;