linux-kernel - Re: [PATCH v2 22/32] s390: define __smp

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Date:	Tue, 5 Jan 2016 16:39:37 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v2 22/32] s390: define __smp_xxx

On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote:

> 
> arch/s390/kernel/vdso.c:        smp_mb();
> 
> Looking at
> 	Author: Christian Borntraeger <borntraeger@...ibm.com>
> 	Date:   Fri Sep 11 16:23:06 2015 +0200
> 
> 	    s390/vdso: use correct memory barrier
> 
> 	    By definition smp_wmb only orders writes against writes. (Finish all
> 	    previous writes, and do not start any future write). To protect the
> 	    vdso init code against early reads on other CPUs, let's use a full
> 	    smp_mb at the end of vdso init. As right now smp_wmb is implemented
> 	    as full serialization, this needs no stable backport, but this change
> 	    will be necessary if we reimplement smp_wmb.
> 
> ok from hypervisor point of view, but it's also strange:
> 1. why isn't this paired with another mb somewhere?
>    this seems to violate barrier pairing rules.
> 2. how does smp_mb protect against early reads on other CPUs?
>    It normally does not: it orders reads from this CPU versus writes
>    from same CPU. But init code does not appear to read anything.
>    Maybe this is some s390 specific trick?
> 
> I could not figure out the above commit.

It was probably me misreading the code. I change a wmb into a full mb here
since I was changing the defintion of wmb to a compiler barrier. I tried to
fixup all users of wmb that really pair with other code. I assumed that there
must be some reader (as there was a wmb before) but I could not figure out
which. So I just played safe here.

But it probably can be removed.

> arch/s390/kvm/kvm-s390.c:       smp_mb();

This can go. If you have a patch, I can carry that via the kvms390 tree,
or I will spin a new patch with you as suggested-by.

Christian

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