[llvm-dev] Unstable XRay test on ARM

[Serge Rogatch via llvm-dev]

Hi,

Indeed, it seems that exactly this happens: "The weak forms (1-2) of the
functions are allowed to fail spuriously, that is, act as if *this !=
 expected even if they are equal." (from
http://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange ). Because
the value returned indicates that they are indeed equal. The reference
doesn't say it shouldn't happen in single-threaded scenario, so I don't see
a reason to investigate why this happens except curiosity... maybe the
mutex is not ready, or CPU cache problem, or it translates into a single
CPU instruction that fails for hardware reasons, who knows.

Dean, is there any reason not to change this to compare_exchange_strong() ?
As I understand, polling in a loop is not an option because it's not clear
for how long to iterate.

Cheers,
Serge

On 25 January 2017 at 18:23, David Blaikie <dblaikie at gmail.com> wrote:

> +Dean Michael Berris <dberris at google.com>
>
>
> On Wed, Jan 25, 2017 at 7:01 AM Oleg Ranevskyy via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi Renato, Dean, Serge,
>>
>> Just looked into the code and wanted to share some thoughts.
>>
>> This might be a compare_exchange_weak spurious failure. ARM is a weakly
>> ordered CPU, but I am not sure whether spurious failures are really
>> possible in a single threaded app. On the other hand, there is no other way
>> for FDRLogging_init to fail in such a way (return XRAY_LOG_UNINITIALIZED
>> instead of XRAY_LOG_INITIALIZED) without any extra output. This is also
>> true for the 2nd test failure in FDRLogging_finalize, which uses a weak
>> exchange too.
>>
>> Probably, the weak exchange needs to be either replaced with a strong one
>> or looped with more detailed CurrentStatus checks.
>>
>> Oleg
>>
>> On Wed, Jan 25, 2017 at 2:02 PM, Renato Golin via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> Hi Dean/Serge,
>>
>> I just spotted this on our bots:
>>
>> First failure, unrelated commit:
>> http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/3190
>>
>> 'XRay-Unit :: unit/XRayFDRLoggingTest/FDRLoggingTest.Simple' FAILED
>> llvm/projects/compiler-rt/lib/xray/tests/unit/fdr_logging_test.cc:55:
>> Failure
>>       Expected: FDRLogging_init(kBufferSize, kBufferMax, &Options,
>> sizeof(FDRLoggingOptions))
>>       Which is: 0
>> To be equal to: XRayLogInitStatus::XRAY_LOG_INITIALIZED
>>       Which is: 2
>> [  FAILED  ] FDRLoggingTest.Simple (0 ms)
>> ...
>>  1 FAILED TEST
>> ==11476==XRay instrumentation map missing. Not initializing XRay.
>>
>> Then a similar, but not identical error:
>> http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/3191
>>
>> 'XRay-Unit :: unit/XRayFDRLoggingTest/FDRLoggingTest.Simple' FAILED
>> llvm/projects/compiler-rt/lib/xray/tests/unit/fdr_logging_test.cc:58:
>> Failure
>>       Expected: FDRLogging_finalize()
>>       Which is: 2
>> To be equal to: XRayLogInitStatus::XRAY_LOG_FINALIZED
>>       Which is: 4
>> [  FAILED  ] FDRLoggingTest.Simple (0 ms)
>> ...
>>  1 FAILED TEST
>> ==11476==XRay instrumentation map missing. Not initializing XRay.
>>
>> Note 0->2 on the first, 2->4 on the second.
>>
>> And then, for no reason, it's green again:
>> http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/3192
>>
>> Looks like an unstable test to me. :)
>>
>> Can you guys have a look?
>>
>> Thanks!
>> --renato
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