From: peterz@infradead•org (Peter Zijlstra)
To: linux-arm-kernel@lists•infradead.org
Subject: [RFC] arm64: Enforce observed order for spinlock and data
Date: Wed, 5 Oct 2016 17:10:57 +0200 [thread overview]
Message-ID: <20161005151057.GJ3142@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <54b18de220f61747bf1c3cdf2a1b9447@codeaurora.org>
On Wed, Oct 05, 2016 at 10:55:57AM -0400, bdegraaf at codeaurora.org wrote:
> On 2016-10-04 15:12, Mark Rutland wrote:
> >Hi Brent,
> >
> >Could you *please* clarify if you are trying to solve:
> >
> >(a) a correctness issue (e.g. data corruption) seen in practice.
> >(b) a correctness issue (e.g. data corruption) found by inspection.
> >(c) A performance issue, seen in practice.
> >(d) A performance issue, found by inspection.
> >
> >Any one of these is fine; we just need to know in order to be able to
> >help effectively, and so far it hasn't been clear.
Brent, you forgot to state which: 'a-d' is the case here.
> I found the problem.
>
> Back in September of 2013, arm64 atomics were broken due to missing barriers
> in certain situations, but the problem at that time was undiscovered.
>
> Will Deacon's commit d2212b4dce596fee83e5c523400bf084f4cc816c went in at
> that
> time and changed the correct cmpxchg64 in lockref.c to cmpxchg64_relaxed.
>
> d2212b4 appeared to be OK at that time because the additional barrier
> requirements of this specific code sequence were not yet discovered, and
> this change was consistent with the arm64 atomic code of that time.
>
> Around February of 2014, some discovery led Will to correct the problem with
> the atomic code via commit 8e86f0b409a44193f1587e87b69c5dcf8f65be67, which
> has an excellent explanation of potential ordering problems with the same
> code sequence used by lockref.c.
>
> With this updated understanding, the earlier commit
> (d2212b4dce596fee83e5c523400bf084f4cc816c) should be reverted.
>
> Because acquire/release semantics are insufficient for the full ordering,
> the single barrier after the store exclusive is the best approach, similar
> to Will's atomic barrier fix.
This again does not in fact describe the problem.
What is the problem with lockref, and how (refer the earlier a-d
multiple choice answer) was this found.
Now, I have been looking, and we have some idea what you _might_ be
alluding to, but please explain which accesses get reordered how and
cause problems.
next prev parent reply other threads:[~2016-10-05 15:10 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-30 17:40 [RFC] arm64: Enforce observed order for spinlock and data Brent DeGraaf
2016-09-30 18:43 ` Robin Murphy
2016-10-01 15:45 ` bdegraaf at codeaurora.org
2016-09-30 18:52 ` Peter Zijlstra
2016-09-30 19:05 ` Peter Zijlstra
2016-10-01 15:59 ` bdegraaf at codeaurora.org
2016-09-30 19:32 ` Mark Rutland
2016-10-01 16:11 ` bdegraaf at codeaurora.org
2016-10-01 18:11 ` Mark Rutland
2016-10-03 19:20 ` bdegraaf at codeaurora.org
2016-10-04 6:50 ` Peter Zijlstra
2016-10-04 10:12 ` Mark Rutland
2016-10-04 17:53 ` bdegraaf at codeaurora.org
2016-10-04 18:28 ` bdegraaf at codeaurora.org
2016-10-04 19:12 ` Mark Rutland
2016-10-05 14:55 ` bdegraaf at codeaurora.org
2016-10-05 15:10 ` Peter Zijlstra [this message]
2016-10-05 15:30 ` bdegraaf at codeaurora.org
2016-10-12 20:01 ` bdegraaf at codeaurora.org
2016-10-13 11:02 ` Will Deacon
2016-10-13 20:00 ` bdegraaf at codeaurora.org
2016-10-14 0:24 ` Mark Rutland
2016-10-05 15:11 ` bdegraaf at codeaurora.org
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