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Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions
- To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 11 Feb 2015 15:28:42 -0800
- Cc: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, KVM list <kvm@xxxxxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Jason Wang <jasowang@xxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Peter Anvin <hpa@xxxxxxxxx>, Andi Kleen <ak@xxxxxxxxxxxxxxx>, Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Christian Borntraeger <borntraeger@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>, Davidlohr Bueso <dave@xxxxxxxxxxxx>, Sasha Levin <sasha.levin@xxxxxxxxxx>, Dave Jones <davej@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Waiman Long <waiman.long@xxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 11 Feb 2015 23:28:59 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On Feb 11, 2015 3:15 PM, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:
>
> Right now it needs to be a locked operation to prevent read-reordering.
> x86 memory ordering rules state that all writes are seen in a globally
> consistent order, and are globally ordered wrt reads *on the same
> addresses*, but reads to different addresses can be reordered wrt to writes.
The modern x86 rules are actually much tighter than that.
Every store is a release, and every load is an acquire. So a non-atomic store is actually a perfectly fine unlock. All preceding stores will be seen by other cpu's before the unlock, and while reads can pass stores, they only pass *earlier* stores.
For *taking* a lock you need an atomic access, because otherwise loads inside the locked region could bleed out to before the store that takes the lock.
ÂÂÂÂ Linus
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