linux-kernel - Re: [PATCH 09/11] job control: reorganize wait_task

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Date:	Wed, 11 May 2011 17:48:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu
Subject: Re: [PATCH 09/11] job control: reorganize wait_task_stopped()

On 05/08, Tejun Heo wrote:
>
> wait_task_stopped() tested task_stopped_code() without acquiring
> siglock and, if stop condition existed, called wait_task_stopped() and
> directly returned the result.
>
>   it may race against SIGCONT generation.

Hmm. This is the plain bug, even if unlikely and minor.

>   It seems that WNOHANG wait correctness has never been guaranteed and
>   everybody has been happy with it for very long time.

Yes, the window is tiny. May be it was never noticed or never
reported because this is hard to diagnose/reproduced.

> As such,
>   although this reorganization improves the situation a bit, I don't
>   consider this to be a bug fix.

But it is?

Can't we push this patch ahead of these changes? I can merge it into
ptrace branch.


>  static int wait_task_stopped(struct wait_opts *wo,
>  				int ptrace, struct task_struct *p)
> @@ -1397,6 +1409,9 @@ static int wait_task_stopped(struct wait_opts *wo,
>  	if (!ptrace && !(wo->wo_flags & WUNTRACED))
>  		return 0;
>
> +	if (!task_stopped_code(p, ptrace))
> +		return 0;
> +
>  	exit_code = 0;
>  	spin_lock_irq(&p->sighand->siglock);
>
> @@ -1607,8 +1622,9 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
>  	 * Wait for stopped.  Depending on @ptrace, different stopped state
>  	 * is used and the two don't interact with each other.
>  	 */
> -	if (task_stopped_code(p, ptrace))
> -		return wait_task_stopped(wo, ptrace, p);
> +	ret = wait_task_stopped(wo, ptrace, p);
> +	if (ret)
> +		return ret;

Reviewed-by: Oleg Nesterov <oleg@...hat.com>

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