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Subject: Re: [PATCH] kvm: fix waitqueue_active without memory barrier in
 virt/kvm/async_pf.c
To: Peter Zijlstra <peterz@infradead.org>
References: <17EC94B0A072C34B8DCF0D30AD16044A028747C1@BPXM09GP.gisp.nec.co.jp>
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 <20151009085040.GZ7520@twins.programming.kicks-ass.net>
Cc: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>, Gleb Natapov <gleb@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <561795CB.30006@redhat.com>
Date: Fri, 9 Oct 2015 12:24:11 +0200
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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