linux-kernel - Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of ->signal to check the task is alive

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Message-ID: <20090205225944.GA10345@redhat.com>
Date:	Thu, 5 Feb 2009 23:59:44 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Lin Ming <ming.m.lin@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of
	->signal to check the task is alive

On 02/05, Roland McGrath wrote:
>
> > We can't use them as refcounts. You can't bump ->live or ->count without
> > breaking group_dead or exec logic. Perhaps we can use ->count, but then
> > we need other changes.
>
> We certainly need to clean up exec anyway.

Agreed.

> > The goal is to keep task->signal after release_task(), it will be freed
> > by __put_task_struct(). This allows a lot of simplifications and we can
> > move some fields from task_struct to signal_struct.
>
> That sounds fine to me in theory, but I still wonder what the story will be
> about the use of siglock.

I think we should change nothing with the usage of siglock for now?

> > But first we should change the code which does [...]
>
> I did understand the rationale given the signal_struct lifetime change.

Ah, sorry for noise then.

> > Even cpu_clock_sample_group() is not safe, unless we add other changes.
>
> Why?  It does no locking and only relies on the signal_struct lifetime.

Yes, I was wrong, thanks. I forgot we should always have a reference
to task_struct anyway.

Oleg.

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