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Date: Thu, 11 Aug 2016 11:31:06 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Susanne Spraul <1vier1@web.de>, parri.andrea@gmail.com
Subject: Re: spin_lock implicit/explicit memory barrier
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On Thu, 11 Aug 2016, Peter Zijlstra wrote:

>On Wed, Aug 10, 2016 at 04:29:22PM -0700, Davidlohr Bueso wrote:
>
>> (1) As Manfred suggested, have a patch 1 that fixes the race against mainline
>> with the redundant smp_rmb, then apply a second patch that gets rid of it
>> for mainline, but only backport the original patch 1 down to 3.12.
>
>I have not followed the thread closely, but this seems like the best
>option. Esp. since 726328d92a42 ("locking/spinlock, arch: Update and fix
>spin_unlock_wait() implementations") is incomplete, it relies on at
>least 6262db7c088b ("powerpc/spinlock: Fix spin_unlock_wait()") to sort
>PPC.

Yeah, and we'd also need the arm bits; which reminds me, aren't alpha
ldl_l/stl_c sequences also exposed to this delaying of the publishing
when a non-owner peeks at the lock? Right now sysv sem's would be busted
when doing either is_locked or unlock_wait, shouldn't these be pimped up
to full smp_mb()s?

Thanks,
Davidlohr