Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

[Ingo Molnar]

* David Howells <dhowells@xxxxxxxxxx> wrote:

> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> 
> > We have atomic_cmpxchg. Can you use that for a sufficient generic
> > implementation?
> 
> No. CMPXCHG/CAS is not as available as XCHG, and it's also unnecessary.

take a look at the PREEMPT_RT implementation of mutexes: it uses 
cmpxchg(), and thus both the down() and the up() fastpath is lockless!  
(And that is a mutex type that does alot more things, as it supports 
priority inheritance.)

architectures which dont have cmpxchg can use a spinlock just fine.

	Ingo
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