linux-kernel - Re: [PATCH 0/3] cgroup: fix race between cgroup_post_fork() and cgroup

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Date:	Mon, 30 Nov 2015 16:16:47 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Zefan Li <lizefan@...wei.com>
Cc:	Aleksa Sarai <cyphar@...har.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] cgroup: fix race between cgroup_post_fork() and
	cgroup_migrate()

On 11/28, Zefan Li wrote:
>
> On 2015/11/28 2:57, Oleg Nesterov wrote:
>> On 11/26, Oleg Nesterov wrote:
>>>
>>> OK. I do not know what exactly do you mean, perhaps if you fix this problem
>>> the race between fork and attach goes away and in this case the fix I sent
>>> is not needed?
>>
>> Otherwise please consider this series.
>>
>> Slightly tested, seems to work; please review.
>>
>
> Allowing tasks migrating between cgroups while forking is problematic. I'm more
> than glad to see those changes.

Yes, I think this way we can probably do other cleanups/fixes in the generic code
too.

For example, cgroup_enable_task_cg_lists() looks racy, spin_lock_irq(siglock)
can't ensure we can't miss PF_EXITING, exit_signals() doesn't take this lock
in the single-threaded case. We can change it to use cgroup_threadgroup_rwsem
and avoid tasklist_lock afaics.

And I forgot to mention, if we apply this series, we need to rename
threadgroup_change_begin/end and remove the "task_struct *tsk" argument, plus
provide the helpers for write_lock/unlock. Then we can remove
uprobe_start_dup_mmap() and change register_for_each_vma() to use the same
lock.

But I failed to invent the good names for the new helpers ;)

> Acked-by: Zefan Li <lizefan@...wei.com>

Thanks!

Oleg.

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