Messages in this thread | | | Date | Fri, 22 Apr 2016 17:53:11 +0100 | From | Will Deacon <> | Subject | Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait() |
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On Sat, Apr 23, 2016 at 12:08:57AM +0800, Boqun Feng wrote: > 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.
Maybe, but that makes it difficult for me to use a load-acquire instruction for the ACQUIRE case.
Will
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