io_uring: ensure workqueue offload grabs ring mutex for poll list

A previous commit moved the locking for the async sqthread, but didn't
take into account that the io-wq workers still need it. We can't use
req->in_async for this anymore as both the sqthread and io-wq workers
set it, gate the need for locking on io_wq_current_is_worker() instead.

Fixes: 8a4955ff ("io_uring: sqthread should grab ctx->uring_lock for submissions")
Reported-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3286,10 +3286,19 @@ static int io_issue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		return ret;
 
 	if (ctx->flags & IORING_SETUP_IOPOLL) {
+		const bool in_async = io_wq_current_is_worker();
+
 		if (req->result == -EAGAIN)
 			return -EAGAIN;
 
+		/* workqueue context doesn't hold uring_lock, grab it now */
+		if (in_async)
+			mutex_lock(&ctx->uring_lock);
+
 		io_iopoll_req_issued(req);
+
+		if (in_async)
+			mutex_unlock(&ctx->uring_lock);
 	}
 
 	return 0;