[PATCH] mm: slub: Ensure that slab_unlock() is atomic

[Peter Zijlstra]

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..