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

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Message-ID: <20151112150251.GZ3972@linux.vnet.ibm.com>
Date:	Thu, 12 Nov 2015 07:02:51 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	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,
	will.deacon@....com, 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 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.

							Thanx, Paul

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