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

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Date:	Wed, 9 Mar 2016 16:42:43 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Wednesday 09 March 2016 04:01 PM, Peter Zijlstra wrote:
> 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..

Indeed I thought about this when we first did the SMP port. The only issue was
somewhat more generated code with the hashed spinlocks (vs. my dumb 2 spin locks -
which as I see now will also cause false sharing - likely ending up in the same
cache line), but I was more of a micro-optimization freak then than I'm now :-)

So yeah I agree !