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Date: Wed, 4 Feb 2009 14:19:01 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Lin Ming <ming.m.lin@intel.com>, Roland McGrath <roland@redhat.com>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of
	->signal to check the task is alive
Message-ID: <20090204131901.GA7367@redhat.com>
References: <20090203231717.GA5028@redhat.com> <1233746479.5076.6.camel@laptop>
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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.