Re: [PATCH for-5.15] io_uring: fix lacking of protection for compl_nr

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

On 8/20/21 7:40 PM, Hao Xu wrote:
> coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
> may cause problems when accessing it parallelly.

Did you hit any problem? It sounds like it should be fine as is:

The trick is that it's only responsible to flush requests added
during execution of current call to tctx_task_work(), and those
naturally synchronised with the current task. All other potentially
enqueued requests will be of someone else's responsibility.

So, if nobody flushed requests, we're finely in-sync. If we see
0 there, but actually enqueued a request, it means someone
actually flushed it after the request had been added.

Probably, needs a more formal explanation with happens-before
and so.

> 
> Fixes: d10299e14aae ("io_uring: inline struct io_comp_state")

FWIW, it came much earlier than this commit, IIRC

commit 2c32395d8111037ae2cb8cab883e80bcdbb70713
Author: Pavel Begunkov <asml.silence@gmail.com>
Date:   Sun Feb 28 22:04:53 2021 +0000

    io_uring: fix __tctx_task_work() ctx race


> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io_uring.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index c755efdac71f..420f8dfa5327 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2003,11 +2003,10 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx)
>  {
>  	if (!ctx)
>  		return;
> -	if (ctx->submit_state.compl_nr) {
> -		mutex_lock(&ctx->uring_lock);
> +	mutex_lock(&ctx->uring_lock);
> +	if (ctx->submit_state.compl_nr)
>  		io_submit_flush_completions(ctx);
> -		mutex_unlock(&ctx->uring_lock);
> -	}
> +	mutex_unlock(&ctx->uring_lock);
>  	percpu_ref_put(&ctx->refs);
>  }
>  
> 

-- 
Pavel Begunkov