Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

[Will Deacon]

On Wed, Apr 13, 2016 at 02:52:43PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 12, 2016 at 05:59:41PM +0100, Will Deacon wrote:
> > Thanks for looking at this!
> 
> n/p, had to se what it would look like etc.. :-)
> 
> > > I've misplaced my arm64 compiler, so this is not even compile tested.
> > 
> > Guess what? ;)
> > 
> 
> > make: *** [init] Error 2
> > make: *** Waiting for unfinished jobs....
> > 
> > (and lot of similar errors).
> > 
> > Looks like you're just missing an #undef in cmpxchg.h.
> > 
> > FWIW, you can pick up arm64 toolchain binaries from:
> > 
> >   https://releases.linaro.org/components/toolchain/binaries/latest-5/
> 
> Ah, I usually build a whole set from sources; for some reason arm64
> didn't build in the latest run. I'll have to kick it.
> 
> > > +#define __CMPWAIT_GEN(w, sz, name)					\
> > > +void __cmpwait_case_##name(volatile void *ptr, unsigned long val)	\
> > > +{									\
> > > +	unsigned long tmp;						\
> > > +									\
> > > +	asm volatile(							\
> > > +	"	ldxr" #sz "\t%" #w "[tmp], %[v]\n"			\
> > > +	"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
> > > +	"	cbnz	%" #w "[tmp], 1f\n"				\
> > 
> > Shouldn't this be cbz? (i.e. branch over the wfe if the value is equal
> > to what we wanted?).
> 
> Indeed so.
> 
> > > +	"	wfe\n"							\
> > > +	"1:"								\
> > > +	: [tmp] "=&r" (tmp), [val] "=&r" (val),				\
> > 
> > We only read val, so it can be an input operand, no?
> 
> True.. :-)
> 
> > > +#define cmpwait(ptr, val) \
> > > +	__cmpwait((ptr), (unsigned long)(val), sizeof(*(ptr)))
> > 
> > We might want to call this cmpwait_relaxed, in case we decide to add
> > fenced versions in the future. Or just make it cmpwait_acquire and
> > remove the smp_rmb() from smp_cond_load_acquire(). Dunno.
> 
> This is something I'll very much leave up to you. I have no idea on the
> tradeoffs involved here.

FWIW, here's a fixup patch to get this patch building and running. I
also noticed some missing casts for the subword cases.

Will

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