Re: possible deadlock in __io_queue_deferred

[Jens Axboe]

On 8/10/20 9:36 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+996f91b6ec3812c48042@xxxxxxxxxxxxxxxxxxxxxxxxx

Thanks, the below should fix this one.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 443eecdfeda9..f9be665d1c5e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -898,6 +898,7 @@ static void io_put_req(struct io_kiocb *req);
 static void io_double_put_req(struct io_kiocb *req);
 static void __io_double_put_req(struct io_kiocb *req);
 static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req);
+static void __io_queue_linked_timeout(struct io_kiocb *req);
 static void io_queue_linked_timeout(struct io_kiocb *req);
 static int __io_sqe_files_update(struct io_ring_ctx *ctx,
 				 struct io_uring_files_update *ip,
@@ -1179,7 +1180,7 @@ static void io_prep_async_link(struct io_kiocb *req)
 			io_prep_async_work(cur);
 }
 
-static void __io_queue_async_work(struct io_kiocb *req)
+static struct io_kiocb *__io_queue_async_work(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_kiocb *link = io_prep_linked_timeout(req);
@@ -1187,16 +1188,19 @@ static void __io_queue_async_work(struct io_kiocb *req)
 	trace_io_uring_queue_async_work(ctx, io_wq_is_hashed(&req->work), req,
 					&req->work, req->flags);
 	io_wq_enqueue(ctx->io_wq, &req->work);
-
-	if (link)
-		io_queue_linked_timeout(link);
+	return link;
 }
 
 static void io_queue_async_work(struct io_kiocb *req)
 {
+	struct io_kiocb *link;
+
 	/* init ->work of the whole link before punting */
 	io_prep_async_link(req);
-	__io_queue_async_work(req);
+	link = __io_queue_async_work(req);
+
+	if (link)
+		io_queue_linked_timeout(link);
 }
 
 static void io_kill_timeout(struct io_kiocb *req)
@@ -1229,12 +1233,19 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
 	do {
 		struct io_defer_entry *de = list_first_entry(&ctx->defer_list,
 						struct io_defer_entry, list);
+		struct io_kiocb *link;
 
 		if (req_need_defer(de->req, de->seq))
 			break;
 		list_del_init(&de->list);
 		/* punt-init is done before queueing for defer */
-		__io_queue_async_work(de->req);
+		link = __io_queue_async_work(de->req);
+		if (link) {
+			__io_queue_linked_timeout(link);
+			/* drop submission reference */
+			link->flags |= REQ_F_COMP_LOCKED;
+			io_put_req(link);
+		}
 		kfree(de);
 	} while (!list_empty(&ctx->defer_list));
 }
@@ -5945,15 +5956,12 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-static void io_queue_linked_timeout(struct io_kiocb *req)
+static void __io_queue_linked_timeout(struct io_kiocb *req)
 {
-	struct io_ring_ctx *ctx = req->ctx;
-
 	/*
 	 * If the list is now empty, then our linked request finished before
 	 * we got a chance to setup the timer
 	 */
-	spin_lock_irq(&ctx->completion_lock);
 	if (!list_empty(&req->link_list)) {
 		struct io_timeout_data *data = &req->io->timeout;
 
@@ -5961,6 +5969,14 @@ static void io_queue_linked_timeout(struct io_kiocb *req)
 		hrtimer_start(&data->timer, timespec64_to_ktime(data->ts),
 				data->mode);
 	}
+}
+
+static void io_queue_linked_timeout(struct io_kiocb *req)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+
+	spin_lock_irq(&ctx->completion_lock);
+	__io_queue_linked_timeout(req);
 	spin_unlock_irq(&ctx->completion_lock);
 
 	/* drop submission reference */

-- 
Jens Axboe