linux-kernel - Re: [PATCH] io_uring: annotate sqd->thread access with data race in cancel path

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Message-ID: <02961c66-19b2-4f55-b785-3c4a132a5da3@gmail.com>
Date: Sat, 11 Jan 2025 12:02:25 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Bui Quang Minh <minhquangbui99@...il.com>, linux-kernel@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
 syzbot+3c750be01dab672c513d@...kaller.appspotmail.com,
 Li Zetao <lizetao1@...wei.com>
Subject: Re: [PATCH] io_uring: annotate sqd->thread access with data race in
 cancel path

On 1/11/25 10:59, Bui Quang Minh wrote:
> The sqd->thread access in io_uring_cancel_generic is just for debug check
> so we can safely ignore the data race.
> 
> The sqd->thread access in io_uring_try_cancel_requests is to check if the
> caller is the sq threadi with the check ctx->sq_data->thread == current. In
> case this is called in a task other than the sq thread, we expect the
> expression to be false. And in that case, the sq_data->thread read can race
> with the NULL write in the sq thread termination. However, the race will
> still make ctx->sq_data->thread == current be false, so we can safely
> ignore the data race.
> 
> Reported-by: syzbot+3c750be01dab672c513d@...kaller.appspotmail.com
> Reported-by: Li Zetao <lizetao1@...wei.com>
> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
> ---
>   io_uring/io_uring.c | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index ff691f37462c..b1a116620ae1 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3094,9 +3094,18 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
>   		ret |= (cret != IO_WQ_CANCEL_NOTFOUND);
>   	}
>   
> -	/* SQPOLL thread does its own polling */
> +	/*
> +	 * SQPOLL thread does its own polling
> +	 *
> +	 * We expect ctx->sq_data->thread == current to be false when
> +	 * this function is called on a task other than the sq thread.
> +	 * In that case, the sq_data->thread read can race with the
> +	 * NULL write in the sq thread termination. However, the race
> +	 * will still make ctx->sq_data->thread == current be false,
> +	 * so we can safely ignore the data race here.
> +	 */
>   	if ((!(ctx->flags & IORING_SETUP_SQPOLL) && cancel_all) ||
> -	    (ctx->sq_data && ctx->sq_data->thread == current)) {
> +	    (ctx->sq_data && data_race(ctx->sq_data->thread) == current)) {
>   		while (!wq_list_empty(&ctx->iopoll_list)) {
>   			io_iopoll_try_reap_events(ctx);
>   			ret = true;

data_race() is a hammer we don't want to use to just silence warnings,
it can hide real problems. The fact that it needs 6 lines of comments
to explain is also not a good sign.

Instead, you can pass a flag, i.e. io_uring_cancel_generic() will have
non zero sqd IFF it's the SQPOLL task.


> @@ -3142,7 +3151,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
>   	s64 inflight;
>   	DEFINE_WAIT(wait);
>   
> -	WARN_ON_ONCE(sqd && sqd->thread != current);

It's not racing if it's the same thread, if it's not it'll trigger
the warning anyway, I don't think we care about this one.

> +	WARN_ON_ONCE(sqd && data_race(sqd->thread) != current);
>   
>   	if (!current->io_uring)
>   		return;

-- 
Pavel Begunkov