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

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Date:	Fri, 13 May 2011 19:21:17 +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()

Hi,

On 05/13, Tejun Heo wrote:
>
> On Thu, May 12, 2011 at 08:33:26PM +0200, Oleg Nesterov wrote:
>
> > At first glance, do_wait() does
> >
> > 	wait_task_stopped();
> > 	wait_task_continued();
> >
> > and the state can be changed CONTINIUED -> STOPPED in between, right?
> > Or something else?
>
> Yeah and exit transitions too.

I am not sure... but probably this depends on definition.

We already checked ->exit_state != ZOMBIE, and we are holding tasklist.
The child can't exit. I mean, it can't change its ->exit_state.

However, SIGKILL can clear SIGNAL_STOP_STOPPED, and we can "miss" it.
But this looks correct, the child is no longer stopped but it is still
not dead. So I think in this case wait(WNOHANG | WEXITED | WSTOPPED)
can fail, notabug.

OTOH, perhaps SIGKILL should set SIGNAL_STOP_CONTINUED in this case?
And keep it if it was already set.

> There simply is no synchronization
> there.  We can probably solve it without acquiring siglock by adding
> "clear this before making state transitions" flag followed by a mb()

perhaps even simpler if ->EXIT transition is fine.

Oleg.

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