Re: [PATCH v2 22/32] s390: define __smp_xxx
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
- To: "Michael S. Tsirkin" <mst@xxxxxxxxxx>, Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
- Subject: Re: [PATCH v2 22/32] s390: define __smp_xxx
- From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
- Date: Tue, 5 Jan 2016 16:39:37 +0100
- Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Arnd Bergmann <arnd@xxxxxxxx>, linux-arch@xxxxxxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, David Miller <davem@xxxxxxxxxxxxx>, linux-ia64@xxxxxxxxxxxxxxx, linuxppc-dev@xxxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, sparclinux@xxxxxxxxxxxxxxx, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx, linux-metag@xxxxxxxxxxxxxxx, linux-mips@xxxxxxxxxxxxxx, x86@xxxxxxxxxx, user-mode-linux-devel@xxxxxxxxxxxxxxxxxxxxx, adi-buildroot-devel@xxxxxxxxxxxxxxxxxxxxx, linux-sh@xxxxxxxxxxxxxxx, linux-xtensa@xxxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, Heiko Carstens <heiko.carstens@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Davidlohr Bueso <dave@xxxxxxxxxxxx>, Andrey Konovalov <andreyknvl@xxxxxxxxxx>
- In-reply-to: <20160105105335-mutt-send-email-mst@redhat.com>
- List-id: <linux-ia64.vger.kernel.org>
- References: <1451572003-2440-1-git-send-email-mst@redhat.com> <1451572003-2440-23-git-send-email-mst@redhat.com> <20160104134525.GA6344@twins.programming.kicks-ass.net> <20160104221323-mutt-send-email-mst@redhat.com> <20160105091319.59ddefc7@mschwide> <20160105105335-mutt-send-email-mst@redhat.com>
- User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0
On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote:
>
> arch/s390/kernel/vdso.c: smp_mb();
>
> Looking at
> Author: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-ia64" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
[Index of Archives]
[Linux Kernel]
[Sparc Linux]
[DCCP]
[Linux ARM]
[Yosemite News]
[Linux SCSI]
[Linux x86_64]
[Linux for Ham Radio]