linux-kernel - Re: [PATCH] block: fix a crash in do_task

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Date:   Thu, 30 May 2019 10:03:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     axboe@...nel.dk, akpm@...ux-foundation.org, hch@....de,
        oleg@...hat.com, gkohli@...eaurora.org, mingo@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: fix a crash in do_task_dead()

On Wed, May 29, 2019 at 04:25:26PM -0400, Qian Cai wrote:

> Fixes: 0619317ff8ba ("block: add polled wakeup task helper")

What is the purpose of that patch ?! The Changelog doesn't mention any
benefit or performance gain. So why not revert that?

> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  include/linux/blkdev.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 592669bcc536..290eb7528f54 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1803,7 +1803,7 @@ static inline void blk_wake_io_task(struct task_struct *waiter)
>  	 * that case, we don't need to signal a wakeup, it's enough to just
>  	 * mark us as RUNNING.
>  	 */
> -	if (waiter == current)
> +	if (waiter == current && in_task())
>  		__set_current_state(TASK_RUNNING);

NAK, No that's broken too.

The right fix is something like:

	if (waiter == current) {
		barrier();
		if (current->state & TASK_NORAL)
			__set_current_state(TASK_RUNNING);
	}

But even that is yuck to do outside of the scheduler code, as it looses
tracepoints and stats.

So can we please just revert that original patch and start over -- if
needed?

>  	else
>  		wake_up_process(waiter);
> -- 
> 1.8.3.1
>