From: Oleg Nesterov <oleg@redhat.com>
To: 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>,
Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group
Date: Tue, 28 Oct 2014 23:52:50 +0100 [thread overview]
Message-ID: <20141028225250.GA8519@redhat.com> (raw)
In-Reply-To: <1414473874.8574.2.camel@tkhai>
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() ?
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.
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.
Oleg.
next prev parent reply other threads:[~2014-10-28 21:56 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 [this message]
2014-10-29 3:20 ` Kirill Tkhai
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20141028225250.GA8519@redhat.com \
--to=oleg@redhat.com \
--cc=burke.libbey@shopify.com \
--cc=ktkhai@parallels.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=tkhai@yandex.ru \
--cc=vdavydov@parallels.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).