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

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Message-ID: <20100611151533.GA2867@redhat.com>
Date:	Fri, 11 Jun 2010 17:15:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Stanislaw Gruszka <sgruszka@...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 06/11, Stanislaw Gruszka wrote:
>
> 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 ...

No, we can't. thread_group_cputimer() does test-and-set ->running
under cputimer->lock.

But when I sent these patches, I realized we have another race here
(with or without these patches). I am already doing the fix.

> > - 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.

Could you clarify?

Oleg.

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