Messages in this thread |  | | Date | Wed, 5 Oct 2016 17:10:57 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] arm64: Enforce observed order for spinlock and data |
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On Wed, Oct 05, 2016 at 10:55:57AM -0400, bdegraaf@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.
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