From: Kirill Tkhai <tkhai@yandex.ru>
To: Oleg Nesterov <oleg@redhat.com>, Kirill Tkhai <ktkhai@parallels.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.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 06:20:48 +0300 [thread overview]
Message-ID: <54505D10.7050809@yandex.ru> (raw)
In-Reply-To: <20141028225250.GA8519@redhat.com>
On 29.10.2014 01:52, Oleg Nesterov wrote:
> On 10/28, Kirill Tkhai wrote:
>>
>> Shouldn't we do that in separate patch? How about this?
>
> Up to Peter, but I think a separate patch is fine.
>
>> [PATCH]sched: Remove lockdep check in sched_move_task()
>>
>> sched_move_task() is the only interface to change sched_task_group:
>> cpu_cgrp_subsys methods and autogroup_move_group() use it.
>
> Yes, but...
>
>> Everything is synchronized by task_rq_lock(), so cpu_cgroup_attach()
>> is ordered with other users of sched_move_task(). This means we do
>> no need RCU here: if we've dereferenced a tg here, the .attach method
>> hasn't been called for it yet.
>>
>> Thus, we should pass "true" to task_css_check() to silence lockdep
>> warnings.
>
> In theory, I am not sure.
>
> However, I never really understood this code and today I forgot everything,
> please correct me.
>
>> @@ -7403,8 +7403,12 @@ void sched_move_task(struct task_struct *tsk)
>> if (unlikely(running))
>> put_prev_task(rq, tsk);
>>
>> - tg = container_of(task_css_check(tsk, cpu_cgrp_id,
>> - lockdep_is_held(&tsk->sighand->siglock)),
>> + /*
>> + * All callers are synchronized by task_rq_lock(); we do not use RCU
>> + * which is pointless here. Thus, we pass "true" to task_css_check()
>> + * to prevent lockdep warnings.
>> + */
>> + tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
>> struct task_group, css);
>
> Why this can't race with cgroup_task_migrate() if it is called by
> cgroup_post_fork() ?
It can race, but which problem is there? The only thing is
cgroup_post_fork()'s or ss->attach()'s call of sched_move_task() will be
NOOP.
cgroup_migrate_add_src()
cgroup_task_migrate()
cgroup_post_fork();
rcu_assign_pointer(tsk->cgroups, new_cset);
sched_move_task();
css->ss->attach(css, &tset);
sched_move_task();
cgroup_migrate_finish()
> 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().
Kirill
next prev parent reply other threads:[~2014-10-29 3:20 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 [this message]
2014-10-29 9:16 ` Peter Zijlstra
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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