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

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Date:	Mon, 19 Jan 2009 02:41:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Menage <menage@...gle.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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


* Paul Menage <menage@...gle.com> wrote:

> On Sun, Jan 18, 2009 at 1:10 AM, Ingo Molnar <mingo@...e.hu> wrote:
> > 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:
> 
> I disagree - cgroup_mutex is a very coarse lock that can be held for 
> pretty long periods of time by the cgroups framework, and should never 
> be part of any fastpath code. So the overhead of a function call should 
> be irrelevant.
> 
> The change that you're proposing would send the message that 
> cgroup_mutex_lock(&cgroup_mutex) is appropriate to use in a 
> performance-sensitive function, when in fact we want to discourage such 
> code from taking this lock and instead use more appropriately 
> fine-grained locks.

Uhm, how does that 'discourage' its use in fastpath code?

It just hides the real lock and invites bad locking/work constructs like 
the one proposed in this thread.

	Ingo
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