linux-kernel - Re: spin_lock implicit/explicit memory barrier

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Message-ID: <20160819140106.GJ10121@twins.programming.kicks-ass.net>
Date:   Fri, 19 Aug 2016 16:01:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Manfred Spraul <manfred@...orfullife.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Susanne Spraul <1vier1@....de>, parri.andrea@...il.com
Subject: Re: spin_lock implicit/explicit memory barrier

On Fri, Aug 12, 2016 at 10:59:46AM +0800, Boqun Feng wrote:
> But if an arch implements its spin_lock() with a full barrier, even
> though the atomic is implemented by ll/sc, the STORE part of which can't
> be reordered with memory operations in the critcal sections. I think
> maybe that's the case for alpha(and also for ARM32).

Correct, Alpha only has a full fence and uses that after the ll/sc to
provide acquire semantics, ARM has other barriers but too uses a full
barrier here.