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

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Message-ID: <20160423040200.GE3369@insomnia>
Date:	Sat, 23 Apr 2016 12:02:00 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, waiman.long@....com,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, dave@...olabs.net
Subject: Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

On Fri, Apr 22, 2016 at 05:53:11PM +0100, Will Deacon wrote:
> 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.
> 

You're right. I was missting that point. Please ignore this proposal.

So I think having a cmpwait_relaxed makes more sense for the people
knowing they could only rely on the control dependency? Or we actually
don't want to encourage this kind of people ;-)

Regards,
Boqun

> Will

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