Re: [patch 1/3] mutex subsystem: trylock

[Nick Piggin]

Ingo Molnar wrote:
> * Nicolas Pitre <nico@xxxxxxx> wrote:

> > +		"1: ldrex	%0, [%3]	\n"
> > +		"subs		%1, %0, #1	\n"
> > +		"strexeq	%2, %1, [%3]	\n"
> > +		"movlt		%0, #0		\n"
> > +		"cmpeq		%2, #0		\n"
> > +		"bgt		1b		\n"
>
>
> so we are back to what is in essence a cmpxchg implementation?

FWIW, I still think we should go for an open-coded "cmpxchg" variant
for UP that disables preempt, and an atomic_cmpxchg variant for SMP.

- good generic implementations
- the UP version is faster than atomic_xchg for non preempt on ARM
- if you really are counting cycles, you'd probably have preempt off
- if you've got preempt on then the preempt_ operations in semaphores
  would be the least of your worries (how about spinlocks?)

Rather than straight out introducing lots of ugliness and complexity
for something that actually slows down the speed critical !preempt
case (but is unlikely to be measurable either way).

--
SUSE Labs, Novell Inc.

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