[patch] mutex: optimise generic mutex implementations

[David Howells]

Nick Piggin <npiggin at suse.de> wrote:

> Speed up generic mutex implementations.
> 
> - atomic operations which both modify the variable and return something imply
>   full smp memory barriers before and after the memory operations involved
>   (failing atomic_cmpxchg, atomic_add_unless, etc don't imply a barrier because
>   they don't modify the target). See Documentation/atomic_ops.txt.
>   So remove extra barriers and branches.
>   
> - All architectures support atomic_cmpxchg. This has no relation to
>   __HAVE_ARCH_CMPXCHG. We can just take the atomic_cmpxchg path unconditionally
> 
> This reduces a simple single threaded fastpath lock+unlock test from 590 cycles
> to 203 cycles on a ppc970 system.
> 
> Signed-off-by: Nick Piggin <npiggin at suse.de>

This seems to work on FRV which uses the mutex-dec generic algorithm, though
you have to take that with a pinch of salt as I don't have SMP hardware for
it.

Acked-by: David Howells <dhowells at redhat.com>