linux-kernel - Re: Behaviour of smp_mb__{before,after}

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Message-ID: <20150113184510.GA31525@redhat.com>
Date:	Tue, 13 Jan 2015 19:45:10 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Will Deacon <will.deacon@....com>
Cc:	paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
	torvalds@...ux-foundation.org, benh@...nel.crashing.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Behaviour of smp_mb__{before,after}_spin* and acquire/release

On 01/13, Will Deacon wrote:
>
>   1. Does smp_mb__before_spinlock actually have to order prior loads
>      against later loads and stores? Documentation/memory-barriers.txt
>      says it does, but that doesn't match the comment

The comment says that smp_mb__before_spinlock() + spin_lock() should
only serialize STOREs with LOADs. This is because it was added to ensure
that the setting of condition can't race with ->state check in ttwu().

But since we use wmb() it obviously serializes STOREs with STORES. I do
not know if this should be documented, but we already have another user
which seems to rely on this fact: set_tlb_flush_pending().

As for "prior loads", this doesn't look true...

Oleg.

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