linux-kernel - Re: [PATCH] mm: slub: Ensure that slab

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Date:	Wed, 9 Mar 2016 11:31:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Christoph Lameter <cl@...ux.com>, linux-mm@...ck.org,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Noam Camus <noamc@...hip.com>, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
	linux-parisc@...r.kernel,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

On Wed, Mar 09, 2016 at 11:13:49AM +0100, Peter Zijlstra wrote:
> ---
> Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock()
> 
> __clear_bit_unlock() is a special little snowflake. While it carries the
> non-atomic '__' prefix, it is specifically documented to pair with
> test_and_set_bit() and therefore should be 'somewhat' atomic.
> 
> Therefore the generic implementation of __clear_bit_unlock() cannot use
> the fully non-atomic __clear_bit() as a default.
> 
> If an arch is able to do better; is must provide an implementation of
> __clear_bit_unlock() itself.
> 

FWIW, we could probably undo this if we unified all the spinlock based
atomic ops implementations (there's a whole bunch doing essentially the
same), and special cased __clear_bit_unlock() for that.

Collapsing them is probably a good idea anyway, just a fair bit of
non-trivial work to figure out all the differences and if they matter
etc..