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

[Jackie Liu <liuyun01@kylinos.cn>]

> 2019年11月21日 09:32，Jackie Liu <liuyun01@kylinos.cn> 写道：
> 
> 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);

There have an problem, need fix.

static int io_req_defer(struct io_kiocb *req)
{
	int ret = __io_req_defer(req);
	if (ret == -EIOCBQUEUED)
		spin_unlock_irq(&ctx->completion_lock);
	return ret;
}

>        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


--
BR, Jackie Liu