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

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Date:	Fri, 9 Oct 2015 10:50:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
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 Fri, Oct 09, 2015 at 10:45:32AM +0200, Paolo Bonzini wrote:
> So you need another smp_mb() after prepare_to_wait().  I'm not sure
> if it's needed also for your original tty report, but I think it is
> for https://lkml.org/lkml/2015/10/8/989 ("mei: fix waitqueue_active
> without memory barrier in mei drivers").
> 
> I wonder if it makes sense to introduce smp_mb__before_spin_lock()
> and smp_mb__after_spin_unlock().  On x86 the former could be a
> simple compiler barrier, and on s390 both of them could.  But that
> should be a separate patch.

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.
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