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runtime: TestLFStackStress fails on darwin/arm on Apple A7 chip #10189

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crawshaw opened this issue Mar 18, 2015 · 6 comments
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runtime: TestLFStackStress fails on darwin/arm on Apple A7 chip #10189

crawshaw opened this issue Mar 18, 2015 · 6 comments
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@crawshaw
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$ go test -cpu=2 -run=TestLFStackStress runtime
--- FAIL: TestLFStackStress-2 (0.58s)
lfstack_test.go:128: Wrong number of nodes 3/100
FAIL

There is also a -cpu=2 channel test failure (TestMultiConsumer), so to rule out the wait channel in the test I tried, rather crudely, swapping it out for a WaitGroup. With the WaitGroup it gives exactly the same error as above, which points in the direction of atomicload64 / cas64 inside the lfstack implementation.
@randall77
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@dvyukov

@crawshaw
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This test passes on an iPhone 4S (dual core Cortex-A9).
It fails on an iPad Mini 3 (dual core 64-bit Apple-A7).

Perhaps Apple broke some locking assumption when they implemented ARMv7 support on their ARMv8 chip.

This doesn't matter for iOS apps, as the App Store requires shipping darwin/arm64 binaries, and those are the binaries that will be used on those chips. For now I'll keep using the iPhone 4S as the darwin/arm builder.

@rsc
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rsc commented Apr 10, 2015

Is this using the atomics in runtime/atomic_arm.go that are implemented with locks? It's a little hard to believe that locks are broken and this is the only thing that fails. But I don't see any non-lock version for 32-bit arm.

@rsc rsc changed the title runtime: darwin/arm lfstack test failure runtime: TestLFStackStress fails on darwin/arm on Apple A7 chip Apr 10, 2015
@rsc rsc added this to the Unplanned milestone Apr 10, 2015
@dvyukov
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dvyukov commented Apr 10, 2015

Mutexes use cas function in lock. On linux/arm we use kernel-provided cas implementation that contains memory barriers if necessary. On darwin/arm we use our own implementation that misses memory barriers. So mutexes can well be broken.

@crawshaw
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Several stdlib tests fail on this device, this was just the simplest of them.

@gopherbot
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CL https://golang.org/cl/12950 mentions this issue.

@rsc rsc closed this as completed in 4bd8040 Jul 30, 2015
@mikioh mikioh modified the milestones: Go1.5, Unplanned Jul 31, 2015
@golang golang locked and limited conversation to collaborators Aug 5, 2016
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@mikioh @rsc @crawshaw @dvyukov @randall77 @gopherbot