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SubjectRe: [patch 0/8] mutex subsystem, ANNOUNCE
On Wed, 21 Dec 2005, Ingo Molnar wrote:

> Changes since the previous version:
>
> - removed the fastpath dependency on __HAVE_ARCH_CMPXCHG: now every
> architecture is able to use the generic mutex_lock/mutex_unlock
> lockless fastpath. The quality of the fastpath is still as good as in
> the previous version.
>
> - added ARCH_IMPLEMENTS_MUTEX_FASTPATH for architectures that want to
> hand-code their own fastpath. The mutex_lock_slowpath,
> mutex_unlock_slowpath and mutex_lock_interruptible_slowpath global
> functions can be used by such architectures in this case, and they
> should implement the mutex_lock(), mutex_unlock() and
> mutex_lock_interruptible() functions themselves. I have tested this
> mechanism on x86. (but x86 wants to use the generic functions
> otherwise, so those changes are not included in this patchqueue.)

This is a good step in the right direction for ARM, but not quite there
yet.

As it is, the core mutex code is still relying on atomic
decrement/increment to work properly. What would be extremely
beneficial on ARM is to be able to use (variants of) atomic_xchg
everywhere. And the semantics of a mutex allows that where a semaphore
doesn't (which is why I see big benefits for ARM with mutexes).

I even forsee a fast path implementation on ARMv6 that would use an
hybrid approach which will be less instructions and cycles than a
standard atomic decrement/increment (they are available only on ARM
version 6 and above).

What we'd need is a bit more flexibility but only at the low level. No
need to reimplement the whole of mutex_lock(), mutex_unlock(), and
friends.

Please consider the 3 following patches that already bring an immediate
benefit on ARM, even if the fast path isn't inlined yet.


Nicolas
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