From: Peter Zijlstra <peterz@infradead.org>
To: Kirill Tkhai <tkhai@yandex.ru>
Cc: Oleg Nesterov <oleg@redhat.com>,
Kirill Tkhai <ktkhai@parallels.com>,
linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Burke Libbey <burke.libbey@shopify.com>,
Vladimir Davydov <vdavydov@parallels.com>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group
Date: Wed, 29 Oct 2014 10:16:40 +0100 [thread overview]
Message-ID: <20141029091640.GW3337@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <54505D10.7050809@yandex.ru>
On Wed, Oct 29, 2014 at 06:20:48AM +0300, Kirill Tkhai wrote:
> > And cgroup_task_migrate() can free ->cgroups via call_rcu(). Of course,
> > in practice raw_spin_lock_irq() should also act as rcu_read_lock(), but
> > we should not rely on implementation details.
>
> Do you mean cgroup_task_migrate()->put_css_set_locked()? It's not
> possible there, because old_cset->refcount is lager than 1. We increment
> it in cgroup_migrate_add_src() and real freeing happens in
> cgroup_migrate_finish(). These functions are around task_migrate(), they
> are pair brackets.
>
> > task_group = tsk->cgroups[cpu_cgrp_id] can't go away because yes, if we
> > race with migrate then ->attach() was not called. But it seems that in
> > theory it is not safe to dereference tsk->cgroups.
>
> old_cset can't be freed in cgroup_task_migrate(), so we can safely
> dereference it. If we've got old_cset in
> cgroup_post_fork()->sched_move_task(), the right sched_task_group will
> be installed by attach->sched_move_task().
Would it be fair to summarise your argument thusly:
"Because sched_move_task() is only called from cgroup_subsys methods
the cgroup infrastructure itself holds reference on the relevant
css sets, and therefore their existence is guaranteed."
?
The question then would be how do we guarantee/assert the assumption
that sched_move_task() is indeed only ever called from such a method.
next prev parent reply other threads:[~2014-10-29 9:16 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-27 10:18 [PATCH] sched: Fix race between task_group and sched_task_group Kirill Tkhai
2014-10-27 12:21 ` Peter Zijlstra
2014-10-27 23:04 ` Oleg Nesterov
2014-10-28 5:24 ` Kirill Tkhai
2014-10-28 22:52 ` Oleg Nesterov
2014-10-29 3:20 ` Kirill Tkhai
2014-10-29 9:16 ` Peter Zijlstra [this message]
2014-10-29 11:13 ` Kirill Tkhai
2014-10-29 19:21 ` Oleg Nesterov
2014-11-04 16:07 ` [tip:sched/urgent] sched: Remove lockdep check in sched_move_task () tip-bot for Kirill Tkhai
2014-10-28 11:01 ` [tip:sched/core] sched: Fix race between task_group and sched_task_group tip-bot for Kirill Tkhai
2015-01-26 23:46 ` [PATCH] " Sasha Levin
2015-01-27 8:48 ` Peter Zijlstra
2015-01-27 9:31 ` Peter Zijlstra
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