Messages in this thread | | | Date | Sat, 23 Apr 2016 00:08:57 +0800 | From | Boqun Feng <> | Subject | Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait() |
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On Tue, Apr 12, 2016 at 05:59:41PM +0100, Will Deacon wrote: [...] > > +static inline void __cmpwait(volatile void *ptr, unsigned long val, int size) > > +{ > > + switch (size) { > > + case 1: return __cmpwait_case_1(ptr, val); > > + case 2: return __cmpwait_case_2(ptr, val); > > + case 4: return __cmpwait_case_4(ptr, val); > > + case 8: return __cmpwait_case_8(ptr, val); > > + default: BUILD_BUG(); > > + } > > + > > + unreachable(); > > +} > > + > > +#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. >
How about replace smp_rmb() with a smp_acquire_barrier__after_cmpwait()? This barrier is designed to provide an ACQUIRE ordering when combining a cmpwait() .
And cmpwait() only has minimal ordering guarantee, but if it is actually an ACQUIRE, then the corresponding smp_acquire_barrier__after_cmpwait() is just empty.
We might need this special barrier on ppc, because we can implement it with "isync" given that cmpwait() has control dependency and ctrl+isync is ACQUIRE on ppc.
Thoughts?
Regards, Boqun
> Will [unhandled content-type:application/pgp-signature] | |