Date: Wed, 9 Mar 2016 11:31:43 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Christoph Lameter <cl@linux.com>, linux-mm@kvack.org,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Noam Camus <noamc@ezchip.com>, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org,
        linux-parisc@vger.kernel,
        "James E.J. Bottomley" <jejb@parisc-linux.org>,
        Helge Deller <deller@gmx.de>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic
Message-ID: <20160309103143.GF25010@twins.programming.kicks-ass.net>
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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..