linux-kernel - Re: [PATCH] locking/qspinlock: Use atomic_sub_return_release in queued_spin

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Message-ID: <20160613194523.GA2094@linux-80c1.suse>
Date:	Mon, 13 Jun 2016 12:45:23 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	arnd@...db.de, peterz@...radead.org, waiman.long@...com
Subject: Re: [PATCH] locking/qspinlock: Use atomic_sub_return_release in
 queued_spin_unlock

On Fri, 03 Jun 2016, Pan Xinhui wrote:

>The existing version uses a heavy barrier while only release semantics
>is required. So use atomic_sub_return_release instead.
>
>Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>

I just noticed this change in -tip and, while I know that saving a barrier
in core spinlock paths is perhaps a worthy exception, I cannot help but
wonder if this is the begging of the end for smp__{before,after}_atomic().