Re: [PATCH] io_uring: hold uring_lock to complete faild polled io in io_wq_submit_work()

[Pavel Begunkov <asml.silence@gmail.com>]

On 14/12/2020 15:49, Xiaoguang Wang wrote:
> io_iopoll_complete() does not hold completion_lock to complete polled
> io, so in io_wq_submit_work(), we can not call io_req_complete() directly,
> to complete polled io, otherwise there maybe concurrent access to cqring,
> defer_list, etc, which is not safe. Commit dad1b1242fd5 ("io_uring: always
> let io_iopoll_complete() complete polled io") has fixed this issue, but
> Pavel reported that IOPOLL apart from rw can do buf reg/unreg requests(
> IORING_OP_PROVIDE_BUFFERS or IORING_OP_REMOVE_BUFFERS), so the fix is
> not good.
> 
> Given that io_iopoll_complete() is always called under uring_lock, so here
> for polled io, we can also get uring_lock to fix this issue.

One thing I don't like is that io_wq_submit_work() won't be able to
publish an event while someone polling io_uring_enter(ENTER_GETEVENTS),
that's because both take the lock. The problem is when the poller waits
for an event that is currently in io-wq (i.e. io_wq_submit_work()).
The polling loop will eventually exit, so that's not a deadlock, but
latency,etc. would be huge.

> 
> Fixes: dad1b1242fd5 ("io_uring: always let io_iopoll_complete() complete polled io")
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>  fs/io_uring.c | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index f53356ced5ab..eab3d2b7d232 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6354,19 +6354,24 @@ static struct io_wq_work *io_wq_submit_work(struct io_wq_work *work)
>  	}
>  
>  	if (ret) {
> +		bool iopoll_enabled = req->ctx->flags & IORING_SETUP_IOPOLL;
> +
>  		/*
> -		 * io_iopoll_complete() does not hold completion_lock to complete
> -		 * polled io, so here for polled io, just mark it done and still let
> -		 * io_iopoll_complete() complete it.
> +		 * io_iopoll_complete() does not hold completion_lock to complete polled
> +		 * io, so here for polled io, we can not call io_req_complete() directly,
> +		 * otherwise there maybe concurrent access to cqring, defer_list, etc,
> +		 * which is not safe. Given that io_iopoll_complete() is always called
> +		 * under uring_lock, so here for polled io, we also get uring_lock to
> +		 * complete it.
>  		 */
> -		if (req->ctx->flags & IORING_SETUP_IOPOLL) {
> -			struct kiocb *kiocb = &req->rw.kiocb;
> +		if (iopoll_enabled)
> +			mutex_lock(&req->ctx->uring_lock);
>  
> -			kiocb_done(kiocb, ret, NULL);
> -		} else {
> -			req_set_fail_links(req);
> -			io_req_complete(req, ret);
> -		}
> +		req_set_fail_links(req);
> +		io_req_complete(req, ret);
> +
> +		if (iopoll_enabled)
> +			mutex_unlock(&req->ctx->uring_lock);
>  	}
>  
>  	return io_steal_work(req);
> 

-- 
Pavel Begunkov