linux-kernel - Re: [PATCH v2 3/6] cgroup: cgroup v2 freezer

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Message-ID: <20181113205831.GP2509588@devbig004.ftw2.facebook.com>
Date:   Tue, 13 Nov 2018 12:58:31 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Roman Gushchin <guroan@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 3/6] cgroup: cgroup v2 freezer

On Tue, Nov 13, 2018 at 08:55:11PM +0000, Roman Gushchin wrote:
> > > > > +	if (lock_task_sighand(task, &flags)) {
> > > > > +		if (test_bit(CGRP_FREEZE, &dst->flags))
> > > > > +			task->jobctl |= JOBCTL_TRAP_FREEZE;
> > > > > +		else
> > > > > +			task->jobctl &= ~JOBCTL_TRAP_FREEZE;
> > > > 
> > > > How are these flags synchronized?
> > > 
> > > Using the css_set_lock.
> > 
> > But other JOBCTL_TRAP bits aren't synchronized by css_set_lock, right?
> 
> But if we don't touch this bit anywhere else, should be fine, right?

But other JOBCTL_TRAP_ bits aren't synchronized with css_set_lock, so
they can be updated (read-modify-write) concurrently and clobber each
other, no?

Thanks.

-- 
tejun