Re: [PATCH 25/33] aio: use xchg() instead of completion_lock

[Theodore Ts'o]

On Thu, Mar 21, 2013 at 09:35:46AM -0700, Kent Overstreet wrote:
> So, for sticking kiocb completions on the kioctx ringbuffer, we need a
> lock - it unfortunately can't be lockless.
> 
> When the kioctx is shared between threads on different cpus and the rate
> of completions is high, this lock sees quite a bit of contention - in
> terms of cacheline contention it's the hottest thing in the aio subsystem.
> 
> That means, with a regular spinlock, we're going to take a cache miss to
> grab the lock, then another cache miss when we touch the data the lock
> protects - if it's on the same cacheline as the lock, other cpus spinning
> on the lock are going to be pulling it out from under us as we're using
> it.
> 
> So, we use an old trick to get rid of this second forced cache miss - make
> the data the lock protects be the lock itself, so we grab them both at
> once.

Reviewed-by: "Theodore Ts'o" <tytso@xxxxxxx>
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