linux-kernel - Re: [PATCH] kvm: fix waitqueue_active without memory barrier in virt/kvm/async

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Date:	Fri, 9 Oct 2015 12:24:11 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kosuke Tatsukawa <tatsu@...jp.nec.com>,
	Gleb Natapov <gleb@...nel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: fix waitqueue_active without memory barrier in
 virt/kvm/async_pf.c



On 09/10/2015 10:50, Peter Zijlstra wrote:
> Not having actually read or thought about the issue at hand, its
> perfectly valid to pair an smp_mb() with either spin_lock() or
> spin_unlock().
> 
> IOW. MB <-> {ACQUIRE, RELEASE} is a valid pairing.

In this case it's an smp_mb() (store-load barrier) being paired with
another smp_mb(), so spin_unlock()'s release is not enough.

Paolo
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