linux-kernel - Re: [PATCH v3] slub: fix false-positive lockdep warning in free

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Date:	Wed, 5 Feb 2014 13:07:05 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Vladimir Davydov <vdavydov@...allels.com>,
	akpm@...ux-foundation.org, penberg@...nel.org, cl@...ux.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl
Subject: Re: [PATCH v3] slub: fix false-positive lockdep warning in
 free_partial()

On Wed, 5 Feb 2014, Steven Rostedt wrote:

> > There's an extremely small overhead of taking this lock, the cache has 
> > been destroyed and is the process of being torn down, there will be 
> > absolutely no contention on n->list_lock.
> 
> But why add it if it isn't necessary? You're even disabling interrupts,
> which means that you add to the response latency. That is, this change
> does affect other aspects of the kernel!
> 

The functions that manipulate the partial lists was modified by 
c65c1877bd68 ("slub: use lockdep_assert_held") which replaced commentary 
with runtime checking on debug kernels with lockdep enabled.  I'm not sure 
adding more code to do the remove_partial() and __remove_partial() variant 
is the right solution to just bypass the check; if anything, I think we 
should accept the fact that the comment should have been "requires 
n->list_lock if the slab cache can be accessed by other cpus" that makes 
it clear we don't need it for init and destroy paths.
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