Date: Wed, 25 Nov 2015 14:51:38 -0500
From: Tejun Heo <tj@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Li Zefan <lizefan@huawei.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH?] race between cgroup_subsys->fork() and cgroup_migrate()
Message-ID: <20151125195138.GC14240@htj.duckdns.org>
References: <20151125163427.GA8139@redhat.com>
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Hello, Oleg.

On Wed, Nov 25, 2015 at 05:34:27PM +0100, Oleg Nesterov wrote:
> IOW. Suppose that the new child is moved right before cgroup_post_fork() does
> 
> 	for_each_subsys_which(...)
> 		ss->fork(child);
> 
> doesn't this mean that after ss->fork() we do the same sequence
> 
> 		pids_uncharge(old_pids, 1);
> 		pids_charge(pids, 1);

You're absolutely right.

> twice? Note that threadgroup_change_begin/end depends on CLONE_THREAD.
> So we can actually hit WARN_ON() in pids_cancel().
> 
> However, we can't simply remove this uncharge/charge afaics. We need this in
> case when the parent was moved to another cgroup before cgroup_post_fork(),
> and then css_set_move_task() moves the child.
> 
> I know almost nothing about cgroups, perhaps I missed something, please
> correct me.

I can't think of anything better than what you're proposing.  Thanks a
lot for tracking it down and fixing it.

> If am right. How about the patch below? percpu_down_read() is cheap. And
> we can simplify cgroup_pids after this change.
> 
> And. We can probably unify cgroup_threadgroup_rwsem and dup_mmap_sem.
> Note that if we take cgroup_threadgroup_rwsem for reading if CLONE_THREAD,
> otherwise we take another percpu-rwsem in dup_mmap(), dup_mmap_sem.

Sounds perfect.  As this needs to go through -stable, can you please
resend the patch with proper description and SOB?  Please also update
the now incorrect comment in can_attach.

Thanks a lot!

-- 
tejun
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