linux-kernel - Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock

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Message-ID: <20160527090548.GZ3193@twins.programming.kicks-ass.net>
Date:	Fri, 27 May 2016 11:05:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net,
	paulmck@...ux.vnet.ibm.com, will.deacon@....com,
	boqun.feng@...il.com, Waiman.Long@....com, tj@...nel.org,
	pablo@...filter.org, kaber@...sh.net, davem@...emloft.net,
	oleg@...hat.com, netfilter-devel@...r.kernel.org,
	sasha.levin@...cle.com, hofrat@...dl.org, rth@...ddle.net,
	vgupta@...opsys.com, linux@...linux.org.uk, realmz6@...il.com,
	rkuo@...eaurora.org, tony.luck@...el.com, james.hogan@...tec.com,
	ralf@...ux-mips.org, dhowells@...hat.com, jejb@...isc-linux.org,
	mpe@...erman.id.au, schwidefsky@...ibm.com,
	ysato@...rs.sourceforge.jp, chris@...kel.net
Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

On Thu, May 26, 2016 at 05:10:36PM -0400, Chris Metcalf wrote:
> On 5/26/2016 10:19 AM, Peter Zijlstra wrote:
> >--- a/arch/tile/lib/spinlock_32.c
> >+++ b/arch/tile/lib/spinlock_32.c
> >@@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
> >  	if (next == curr)
> >  		return;
> >+	smp_rmb();
> >+
> >  	/* Wait until the current locker has released the lock. */
> >  	do {
> >  		delay_backoff(iterations++);
> >  	} while (READ_ONCE(lock->current_ticket) == curr);
> >+
> >+	smp_acquire__after_ctrl_dep();
> >  }
> >  EXPORT_SYMBOL(arch_spin_unlock_wait);
> >--- a/arch/tile/lib/spinlock_64.c
> >+++ b/arch/tile/lib/spinlock_64.c
> >@@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
> >  	if (arch_spin_next(val) == curr)
> >  		return;
> >+	smp_rmb();
> >+
> >  	/* Wait until the current locker has released the lock. */
> >  	do {
> >  		delay_backoff(iterations++);
> >  	} while (arch_spin_current(READ_ONCE(lock->lock)) == curr);
> >+
> >+	smp_acquire__after_ctrl_dep();
> >  }
> >  EXPORT_SYMBOL(arch_spin_unlock_wait);
> 
> The smp_rmb() are unnecessary for tile.  We READ_ONCE next/curr from the
> lock and compare them, so we know the load(s) are complete.  There's no
> microarchitectural speculation going on so that's that.  Then we READ_ONCE
> the next load on the lock from within the wait loop, so our load/load
> ordering is guaranteed.

Does TILE never speculate reads? Because in that case the control
dependency already provides a full load->load,store barrier and you'd
want smp_acquire__after_ctrl_dep() to be a barrier() instead of
smp_rmb().