io_uring: remove poll entry from list when canceling all

[ upstream commmit 61bc84c ]

When the ring is exiting, as part of the shutdown, poll requests are
removed. But io_poll_remove_all() does not remove entries when finding
them, and since completions are done out-of-band, we can find and remove
the same entry multiple times.

We do guard the poll execution by poll ownership, but that does not
exclude us from reissuing a new one once the previous removal ownership
goes away.

This can race with poll execution as well, where we then end up seeing
req->apoll be NULL because a previous task_work requeue finished the
request.

Remove the poll entry when we find it and get ownership of it. This
prevents multiple invocations from finding it.

Fixes: aa43477 ("io_uring: poll rework")
Reported-by: Dylan Yudaken <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
[pavel: backport]
Signed-off-by: Pavel Begunkov <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: axboe

fs/io_uring.c

@@ -5720,6 +5720,7 @@ static bool io_poll_remove_all(struct io_ring_ctx *ctx, struct task_struct *tsk,
 		list = &ctx->cancel_hash[i];
 		hlist_for_each_entry_safe(req, tmp, list, hash_node) {
 			if (io_match_task_safe(req, tsk, cancel_all)) {
+				hlist_del_init(&req->hash_node);
 				io_poll_cancel_req(req);
 				found = true;
 			}