Re: [PATCH 25/32] aio: use xchg() instead of completion_lock

[Andrew Morton]

On Wed, 26 Dec 2012 18:00:04 -0800
Kent Overstreet <koverstreet@xxxxxxxxxx> 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.

Boy I hope you got that right.

Did you consider using bit_spin_lock() on the upper bit of `tail'? 
We've done that in other places and we at least know that it works. 
And it has the optimisations for CONFIG_SMP=n, understands
CONFIG_DEBUG_SPINLOCK, has arch-specific optimisations, etc.
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