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

[Kyle Moffett]

On Dec 16, 2005, at 10:24, Linh Dang wrote:
> Well, it may be true for ARM but for ppc (i dunno what exactly llsc  
> means but someone in the thread put ppc in llsc group)  it's:
>
>    loop:
>         load-reserve foo => old
>         new = old * N
>         store-conditional new => foo
>         if failed goto loop

LLSC == Load-Locked/Store-Conditional.  It's a slightly different  
name for your Load-Reserve/Store-Conditional

You still miss his point.  That is _GOOD_ code.  Russell's point is  
that if somebody does this in generic code:

do {
	old = atomic_read(&foo);
	new = old * 2;
} while (atomic_cmpxchg(&foo, old, new) != old);

On PPC or ARM or another LLSC architecture it does not end up looking  
like the good code, it looks like this (which is clearly inefficient):

> > And for architectures with llsc, this becomes:
> >
> > loop:	load foo => old
> > 	new = old * N
> > loop2:	load locked foo => ret
> > 	compare ret & old
> > 	if equal store conditional new in foo
> > 		if store failed because we lost the lock, goto loop2
> > 	compare ret & old
> > 	if not equal goto loop

Cheers,
Kyle Moffett

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