Re: [RFC PATCH] io_uring: don't submit sqes when ctx->refs is dying

[Jens Axboe <axboe@kernel.dk>]

On 5/13/20 6:37 AM, Xiaoguang Wang wrote:
> When IORING_SETUP_SQPOLL is enabled, io_ring_ctx_wait_and_kill() will wait
> for sq thread to idle by busy loop:
>     while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
>         cond_resched();
> Above codes are not friendly, indeed I think this busy loop will introduce a
> cpu burst in current cpu, though it maybe short.
> 
> In this patch, if ctx->refs is dying, we forbids sq_thread from submitting
> sqes anymore, just discard leftover sqes.

I don't think this really changes anything. What happens if:

> @@ -6051,7 +6053,8 @@ static int io_sq_thread(void *data)
>  		}
>  
>  		mutex_lock(&ctx->uring_lock);
> -		ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
> +		if (likely(!percpu_ref_is_dying(&ctx->refs)))
> +			ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
>  		mutex_unlock(&ctx->uring_lock);
>  		timeout = jiffies + ctx->sq_thread_idle;

You check for dying here, but that could change basically while you're
checking it. So you're still submitting sqes with a ref that's going
away. You've only reduced the window, you haven't eliminated it.

-- 
Jens Axboe