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Subject: Re: Store Buffers (was Re: Is it OK to pass non-acquired objects to
 kfree?)
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From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <55F13387.4030803@suse.cz>
Date: Thu, 10 Sep 2015 09:38:47 +0200
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To: Christoph Lameter <cl@linux.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Andrey Konovalov <andreyknvl@google.com>, Alexander Potapenko <glider@google.com>

On 09/10/2015 03:47 AM, Christoph Lameter wrote:
> On Wed, 9 Sep 2015, Paul E. McKenney wrote:
> 
>> > But then again kfree() contains a barrier() which would block the compiler
>> > from moving anything into the free path.
>>
>> That barrier() is implicit in the fact that kfree() is an external
>> function?  Or are my eyes failing me?

Is the "external function" not enough? Does it change e.g. with LTO, or is that
also subject to the aliasing rules (which I admit not knowing exactly)?

> 
> kfree at some point calls slab_free(). That function has a barrier. All
> free operations go through it.

SLAB doesn't have such barrier AFAICS. It will put the object on per-cpu cache
and that's it. Only flushing the full cache takes a spin lock.

> 
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