linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Message-ID: <20150714101659.GA16213@arm.com>
Date:	Tue, 14 Jul 2015 11:16:59 +0100
From:	Will Deacon <will.deacon@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Ellerman <michaele@....ibm.com>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 13, 2015 at 11:31:29PM +0100, Benjamin Herrenschmidt wrote:
> On Mon, 2015-07-13 at 13:15 +0100, Will Deacon wrote:
> > This didn't go anywhere last time I posted it, but here it is again.
> > I'd really appreciate some feedback from the PowerPC guys, especially as
> > to whether this change requires them to add an additional barrier in
> > arch_spin_unlock and what the cost of that would be.
> 
> We'd have to turn the lwsync in unlock or the isync in lock into a full
> barrier. As it is, we *almost* have a full barrier semantic, but not
> quite, as in things can get mixed up inside spin_lock between the LL and
> the SC (things leaking in past LL and things leaking "out" up before SC
> and then getting mixed up in there).

Thanks, Ben.

> Michael, at some point you were experimenting a bit with that and tried
> to get some perf numbers of the impact that would have, did that
> solidify ? Otherwise, I'll have a look when I'm back next week.

These numbers would be really interesting...

Will
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