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

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Date:	Sat, 23 Apr 2016 04:37:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Will Deacon <will.deacon@....com>, 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 Sat, Apr 23, 2016 at 12:08:57AM +0800, Boqun Feng wrote:
> 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() .

That's a horrible name for a barrier :-)

> 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?

Provide a PPC specific smp_cond_load_acquire() using ISYNC ?