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: <20090204131901.GA7367@redhat.com>
Date:	Wed, 4 Feb 2009 14:19:01 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Lin Ming <ming.m.lin@...el.com>,
	Roland McGrath <roland@...hat.com>,
	"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 02/04, Peter Zijlstra wrote:
>
> On Wed, 2009-02-04 at 00:17 +0100, Oleg Nesterov wrote:
> > Sadly, it is not trivial to audit kernel/posix-cpu-timers.c, but it really
> > abuses tasklist_lock. I believe it doesn't need this lock at all, but the
> > changes are not easy to test.
>
> It uses that to hold of task reaping so ->signal doesn't go away.

Yes sure, but ->siglock alone is enough (this was not true when this code
was written, as far as I know). It is not trivial to remove tasklist
completely, but some places are trivial.

> If we make ->signal refcountable, and rcu freed along with the tasks I
> think we can get away without tasklist_lock.

I think this is possible even without this change (which is good anyway).
But the problem is not only that ->signal can go away. For example,
posix_cpu_timer_set/posix_cpu_timer_schedule should not proceed if the
task was already released, even if it had the valid ->signal.

Oleg.

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