Re: [PATCH] io_uring: fix race with shadow drain deferrals

[Jackie Liu <liuyun01@kylinos.cn>]

2019年11月21日 07:58，Jens Axboe <axboe@kernel.dk> 写道：

> 
> On 11/20/19 4:07 PM, Jens Axboe wrote:
>> When we go and queue requests with drain, we check if we need to defer
>> based on sequence. This is done safely under the lock, but then we drop
>> the lock before actually inserting the shadow. If the original request
>> is found on the deferred list by another completion in the mean time,
>> it could have been started AND completed by the time we insert the
>> shadow, which will stall the queue.
>> 
>> After re-grabbing the completion lock, check if the original request is
>> still in the deferred list. If it isn't, then we know that someone else
>> already found and issued it. If that happened, then our job is done, we
>> can simply free the shadow.
>> 
>> Cc: Jackie Liu <liuyun01@kylinos.cn>
>> Fixes: 4fe2c963154c ("io_uring: add support for link with drain")
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> BTW, the other solution here is to not release the completion_lock if
> we're going to return -EIOCBQUEUED, and let the caller do what it needs
> before releasing it. That'd look something like this, with some sparse
> annotations to keep things happy.
> 
> I think the original I posted here is easier to follow, and the
> deferral list is going to be tiny in general so it won't really add
> any extra overhead.
> 
> Let me know what you think and prefer.
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 6175e2e195c0..0d1f33bcedc0 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2552,6 +2552,11 @@ static int io_async_cancel(struct io_kiocb *req, const struct io_uring_sqe *sqe,
> 	return 0;
> }
> 
> +/*
> + * Returns with ctx->completion_lock held if -EIOCBQUEUED is returned, so
> + * the caller can make decisions based on the deferral without worrying about
> + * the request being found and issued in the mean time.
> + */
> static int io_req_defer(struct io_kiocb *req)
> {
> 	const struct io_uring_sqe *sqe = req->submit.sqe;
> @@ -2579,7 +2584,7 @@ static int io_req_defer(struct io_kiocb *req)
> 
> 	trace_io_uring_defer(ctx, req, false);
> 	list_add_tail(&req->list, &ctx->defer_list);
> -	spin_unlock_irq(&ctx->completion_lock);
> +	__release(&ctx->completion_lock);
> 	return -EIOCBQUEUED;
> }
> 
> @@ -2954,6 +2959,7 @@ static void __io_queue_sqe(struct io_kiocb *req)
> 
> static void io_queue_sqe(struct io_kiocb *req)
> {
> +	struct io_ring_ctx *ctx = req->ctx;
> 	int ret;
> 
> 	ret = io_req_defer(req);
> @@ -2963,6 +2969,9 @@ static void io_queue_sqe(struct io_kiocb *req)
> 			if (req->flags & REQ_F_LINK)
> 				req->flags |= REQ_F_FAIL_LINK;
> 			io_double_put_req(req);
> +		} else {
> +			__acquire(&ctx->completion_lock);
> +			spin_unlock_irq(&ctx->completion_lock);
> 		}
> 	} else
> 		__io_queue_sqe(req);
> @@ -3001,16 +3010,17 @@ static void io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
> 				__io_free_req(shadow);
> 			return;
> 		}
> +		__acquire(&ctx->completion_lock);
> 	} else {
> 		/*
> 		 * If ret == 0 means that all IOs in front of link io are
> 		 * running done. let's queue link head.
> 		 */
> 		need_submit = true;
> +		spin_lock_irq(&ctx->completion_lock);
> 	}
> 
> 	/* Insert shadow req to defer_list, blocking next IOs */
> -	spin_lock_irq(&ctx->completion_lock);
> 	trace_io_uring_defer(ctx, shadow, true);
> 	list_add_tail(&shadow->list, &ctx->defer_list);
> 	spin_unlock_irq(&ctx->completion_lock);

This is indeed a potential lock issue, thanks, I am prefer this solution, clearer than first one.
But It may be a bit difficult for other people who read the code, use 'io_req_defer_may_lock'?

who about this?

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5ad652f..6fdaeb1 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2469,7 +2469,7 @@ static int io_async_cancel(struct io_kiocb *req, const struct io_uring_sqe *sqe,
        return 0;
 }

-static int io_req_defer(struct io_kiocb *req)
+static int __io_req_defer(struct io_kiocb *req)
 {
        const struct io_uring_sqe *sqe = req->submit.sqe;
        struct io_uring_sqe *sqe_copy;
@@ -2495,8 +2495,21 @@ static int io_req_defer(struct io_kiocb *req)

        trace_io_uring_defer(ctx, req, false);
        list_add_tail(&req->list, &ctx->defer_list);
+
+       return -EIOCBQUEUED;
+}
+
+static int io_req_defer(struct io_kiocb *req)
+{
+       int ret = __io_req_defer(req);
        spin_unlock_irq(&ctx->completion_lock);
-       return -EIOCBQUEUED;
+       return ret;
+}
+
+static int io_req_defer_may_lock(struct io_kiocb *req)
+{
+       return __io_req_defer(req);
+
 }

 static int __io_submit_sqe(struct io_kiocb *req, struct io_kiocb **nxt,
@@ -2927,7 +2940,7 @@ static int io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
         * list.
         */
        req->flags |= REQ_F_IO_DRAIN;
-       ret = io_req_defer(req);
+       ret = io_req_defer_may_lock(req);
        if (ret) {
                if (ret != -EIOCBQUEUED) {
                        io_cqring_add_event(req, ret);
@@ -2941,10 +2954,10 @@ static int io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
                 * running done. let's queue link head.
                 */
                need_submit = true;
+               spin_lock_irq(&ctx->completion_lock);
        }

        /* Insert shadow req to defer_list, blocking next IOs */
-       spin_lock_irq(&ctx->completion_lock);
        trace_io_uring_defer(ctx, shadow, true);
        list_add_tail(&shadow->list, &ctx->defer_list);
        spin_unlock_irq(&ctx->completion_lock);

--
BR, Jackie Liu