linux-kernel - Re: [PATCH 4/5] thread_group_cputime: simplify, document the "alive" check

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Message-ID: <20100611143525.GA2586@dhcp-lab-161.englab.brq.redhat.com>
Date:	Fri, 11 Jun 2010 16:35:26 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] thread_group_cputime: simplify, document the
	"alive" check

On Fri, Jun 11, 2010 at 01:09:56AM +0200, Oleg Nesterov wrote:
> thread_group_cputime() looks as if it is rcu-safe, but in fact this
> was wrong until ea6d290c which pins task->signal to task_struct.
> It checks ->sighand != NULL under rcu, but this can't help if ->signal
> can go away. Fortunately the caller either holds ->siglock, or it is
> fastpath_timer_check() which uses current and checks exit_state == 0.

Hmm, I thought we avoided calling thread_group_cputime() from
fastpatch_timer_check(), but seems it is still possible when we
call run_posix_cpu_timers() on two different cpus simultaneously ...

> - Since ea6d290c commit tsk->signal is stable, we can read it first
>   and avoid the initialization from INIT_CPUTIME.
> 
> - Even if tsk->signal is always valid, we still have to check it
>   is safe to use next_thread() under rcu_read_lock(). Currently
>   the code checks ->sighand != NULL, change it to use pid_alive()
>   which is commonly used to ensure the task wasn't unhashed before
>   we take rcu_read_lock().

I'm not sure how important are values of almost dead task, but
perhaps would be better to return times form all threads
using as base sig->curr_target in loop.

Stanislaw
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