linux-kernel - Re: [patch -mm] slab: use CPU_LOCK

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Message-ID: <20070111023005.GA5357@in.ibm.com>
Date:	Thu, 11 Jan 2007 08:00:05 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Gilbert <bgilbert@...cmu.edu>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Gautham shenoy <ego@...ibm.com>, Andrew Morton <akpm@...l.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [patch -mm] slab: use CPU_LOCK_[ACQUIRE|RELEASE]

On Wed, Jan 10, 2007 at 10:20:28AM -0800, Christoph Lameter wrote:
> I have got a bad feeling about upcoming deadlock problems when looking at
> the mutex_lock / unlock code in cpuup_callback in slab.c. Branches
> that just obtain a lock or release a lock? I hope there is some
> control of  what happens between lock acquisition and release?

A cpu hotplug should happen between LOCK_ACQUIRE/RELEASE

> You are aware that this lock is taken for cache shrinking/destroy, tuning
> of cpu cache sizes, proc output and cache creation? Any of those run on
> the same processor should cause a deadlock.

Why? mutex_lock() taken in LOCK_ACQ will just block those functions
(cache create etc) from proceeding simultaneously as a hotplug event.
This per-subsystem mutex_lock() is supposed to be a replacement for the global
lock_cpu_hotplug() lock .. 

But the whole thing is changing again ..we will likely move towards a
process freezer based cpu hotplug locking ..all the lock_cpu_hotplugs()
and the existing LOCK_ACQ/RELS can go away when we do that ..

-- 
Regards,
vatsa
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