linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Date:	Thu, 12 Nov 2015 21:53:04 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Boqun Feng <boqun.feng@...il.com>, Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, corbet@....net, mhocko@...nel.org,
	dhowells@...hat.com, torvalds@...ux-foundation.org,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Thu, Nov 12, 2015 at 07:02:51AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 12, 2015 at 10:49:02PM +0800, Boqun Feng wrote:
> > On Thu, Nov 12, 2015 at 06:40:04AM -0800, Paul E. McKenney wrote:
> > [snip]
> > > 
> > > I cannot resist suggesting that any lock that interacts with
> > > spin_unlock_wait() must have all relevant acquisitions followed by
> > > smp_mb__after_unlock_lock().
> > > 
> > 
> > But
> > 
> > 1.	This would expand the purpose of smp_mb__after_unlock_lock(),
> > 	right? smp_mb__after_unlock_lock() is for making UNLOCK-LOCK
> > 	pair global transitive rather than guaranteeing no operations
> > 	can be reorder before the STORE part of LOCK/ACQUIRE.
> 
> Indeed it would.  Which might be OK.
> 
> > 2.	If ARM64 has the same problem as PPC now,
> > 	smp_mb__after_unlock_lock() can't help, as it's a no-op on
> > 	ARM64.
> 
> Agreed, and that is why we need Will to weigh in.

I really don't want to implement smp_mb__after_unlock_lock, because we
don't need it based on its current definition and I think there's a better
way to fix spin_unlock_wait (see my other post).

Will
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