linux-kernel - Re: compiler bug gcc4.6/4.7 with ACCESS

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Date:	Thu, 20 Nov 2014 12:30:30 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, KVM <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andreas Krebbel <Andreas.Krebbel@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: compiler bug gcc4.6/4.7 with ACCESS_ONCE and workarounds

On Thu, Nov 20, 2014 at 3:39 AM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
>
> So It looks like we could make a change to ACCESS_ONCE. Would something like
>
> CONFIG_ARCH_SCALAR_ACCESS_ONCE be a good start?

No, if it's just a handful of places to be fixed, let's not add config
options for broken cases.

> This would boil down to
> Patch1: Provide stricter ACCESS_ONCE if CONFIG_ARCH_SCALAR_ACCESS_ONCE is set + docu update + comments
> Patch2: Change mm/* to barriers
> Patch3: Change x86 locks
> Patch4: Change x86 gup
> Patch4: Enable CONFIG_ARCH_SCALAR_ACCESS_ONCE for s390x and x86

Just do patches 2-4 first, and then patch 1 unconditionally.

Obviously you'd need to spread the word on linux-arch to see how bad
it is for other cases, but if other architectures are at all like x86
and s390, and just require a few trivial patches, let's not make this
some config option.

                   Linus
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