linux-kernel - Re: Question on smp_mb__before

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Message-ID: <20160906174209.GB29382@arm.com>
Date:   Tue, 6 Sep 2016 18:42:10 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: Question on smp_mb__before_spinlock

On Tue, Sep 06, 2016 at 01:17:53PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 05, 2016 at 11:10:22AM +0100, Will Deacon wrote:
> 
> > > The second issue I wondered about is spinlock transitivity. All except
> > > powerpc have RCsc locks, and since Power already does a full mb, would
> > > it not make sense to put it _after_ the spin_lock(), which would provide
> > > the same guarantee, but also upgrades the section to RCsc.
> > > 
> > > That would make all schedule() calls fully transitive against one
> > > another.
> > 
> > It would also match the way in which the arm64 atomic_*_return ops
> > are implemented, since full barrier semantics are required there.
> 
> Hmm, are you sure; the way I read arch/arm64/include/asm/atomic_ll_sc.h
> is that you do ll/sc-rel + mb.

Yes, all I meant was that we put the full barrier at the end, but the
two things are indeed different sequences.

Will