linux-kernel - Re: [PATCH 1/7] cgroup: cgroup_subsys->fork() should be called after the task is added to css

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Date:	Wed, 24 Oct 2012 15:04:58 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	rjw@...k.pl, linux-kernel@...r.kernel.org, lizefan@...wei.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/7] cgroup: cgroup_subsys->fork() should be called after
 the task is added to css_set

Hello,

On Tue, Oct 23, 2012 at 05:51:28PM +0200, Oleg Nesterov wrote:
> Yes, yes. But in this case (I mean, for uprobes) "threadgroup" in the name
> is misleading. It should be called unconditially without any argument.
> 
> Please see
> 
> 	[PATCH 1/2] brw_mutex: big read-write mutex
> 	http://marc.info/?l=linux-kernel&m=135032816223715

Ooh... that's something completely different.

> 	[PATCH 2/2] uprobes: Use brw_mutex to fix register/unregister vs dup_mmap() race
> 	http://marc.info/?l=linux-kernel&m=135032817823720
> 
> for details, but in short 2/2 needs this giant lock to block dup_mmap()
> system-wide, while cgroup (currently) only needs threadgroup lock if
> CLONE_THREAD (ignoring do_exit) and per-task.
> 
> So please forget, I no longer think it makes sense to use the same
> thing for uprobes and cgroups.

It is quite tempting to reduce hot path overhead and penalize cgroup
migration ops more tho.  Write-locking brw_mutex on migration might
not be too bad.  Why did you change your mind?

Thanks.

--
tejun
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