linux-kernel - Re: [PATCH 1/3] cgroup: convert open-coded mutex_lock(&cgroup_mutex) calls into cgroup

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Message-ID: <20090118091038.GC27144@elte.hu>
Date:	Sun, 18 Jan 2009 10:10:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, menage@...gle.com,
	miaox@...fujitsu.com, maxk@...lcomm.com,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/3] cgroup: convert open-coded
	mutex_lock(&cgroup_mutex) calls into cgroup_lock() calls


* Lai Jiangshan <laijs@...fujitsu.com> wrote:

> Convert open-coded mutex_lock(&cgroup_mutex) calls into cgroup_lock()
> calls and convert mutex_unlock(&cgroup_mutex) calls into cgroup_unlock()
> calls.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> Cc: Max Krasnyansky <maxk@...lcomm.com>
> Cc: Miao Xie <miaox@...fujitsu.com>
> ---

(please include diffstat output in patches, so that the general source 
code impact can be seen at a glance.)

> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index c298310..75a352b 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -616,7 +688,7 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
>  		 * agent */
>  		synchronize_rcu();
>  
> -		mutex_lock(&cgroup_mutex);
> +		cgroup_lock();

this just changes over a clean mutex call to a wrapped lock/unlock 
sequence that has higher overhead in the common case.

We should do the exact opposite, we should change this opaque API:

 void cgroup_lock(void)
 {
         mutex_lock(&cgroup_mutex);
 }

To something more explicit (and more maintainable) like:

  cgroup_mutex_lock(&cgroup_mutex);
  cgroup_mutex_unlock(&cgroup_mutex);

Which is a NOP in the !CGROUPS case and maps to mutex_lock/unlock in the 
CGROUPS=y case.

	Ingo
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