linux-kernel - Re: Q: schedule() and implied barriers on arm64

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Message-ID: <20151028103815.GA29512@arm.com>
Date:	Wed, 28 Oct 2015 10:39:37 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Q: schedule() and implied barriers on arm64

On Tue, Oct 27, 2015 at 07:40:20PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2015 at 04:19:48PM +0000, Will Deacon wrote:
> > ... and the 'normal' code will have a control hazard somewhere, followed
> > by the implicit ISB in exception return, so there's a barrier of sorts
> > there too.
> 
> Which exception return?

The return to userspace after the interrupt/fault/system call that got us
into the kernel.

> > The problem is that people say "full barrier" without defining what it
> > really means, and we end up going round the houses on things like
> > transitivity (which ctrl + isb doesn't always give you).
> 
> I pretty much meant smp_mb() here :-)

In which case, we don't provide the transitivity guarantees that you would
get from an smp_mb().

Will
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