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

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Message-ID: <alpine.DEB.2.10.1402051439100.23701@nuc>
Date:	Wed, 5 Feb 2014 14:42:26 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Vladimir Davydov <vdavydov@...allels.com>, rientjes@...gle.com,
	akpm@...ux-foundation.org, penberg@...nel.org,
	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:

> Really? We are adding a spin lock for a case where it is not needed just to
> quiet lockdep?

Well its a very rarely used code path. Doesnt matter performance wise
since a slab cache should have no objects when its going to be removed.

> Now if it really isn't needed, then why don't we do the following instead of
> adding the overhead of taking a lock?
>
> static inline
> __remove_partial(struct kmem_cache_node *n, struct page *page)
> {
> 	list_del(&page->lru);
> 	n->nr_partial--;
> }
>
> static inline remove_partial(struct kmem_cache_node *n,
> 			     struct page *page)
> {
> 	lockdep_assert_held(&n->list_lock);
> 	__remove_partial(n, page);
> }
>
> And then just call __remove_partial() where we don't need to check if the
> lock is held or not with a big comment to it.
>
> That, IMNSHO, is a much better solution

Ok with me. It just adds another variant of remove_partial.

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