[phobos] std.parallelism's unit tests randomly hang on win32

[David Simcha]

Yes, it works as a single threaded program, but there are a lot of code
paths that are never taken unless a worker thread finishes a job before the
submitter thread needs the result (which obviously can't happen in
single-threaded mode).  Therefore, this does not prove that the issue is a
concurrency bug.

On Wed, May 4, 2011 at 1:37 PM, Walter Bright <walter at digitalmars.com>wrote:

> Does it work as a single threaded program?
>
>
> On 5/4/2011 6:51 AM, David Simcha wrote:
>
>> I went a slightly different route and tried to reduce the problem to as
>> small a test case as possible, like I would normally do for a compiler bug.
>>  So far I've managed to reduce it to ~560 lines.  I've discovered this one's
>> more unstable (i.e. the results change a lot more in response to slight
>> perturbations) than I thought.  Just changing the layout of the Task struct
>> (deleting member variables that are no longer used anywhere) makes it go
>> from unit test failures to access violations. Adding or removing try/catch
>> blocks or empty destructors in some places can completely prevent the bug
>> from manifesting.  On Linux, if I perturb things slightly by changing the
>> layout of Task, I get exceptions thrown from core.sync.
>>
>> This looks like some kind of memory/stack corruption bug but due to its
>> nondeterminism (only a few thread interleavings seem to take the proper
>> codepath and I'm not sure which ones) and its very indirect manifestation
>> (memory corruption; the low order bit overwriting thing was, I think, just a
>> manifestation of a deeper problem), I am somewhat at a loss for how to debug
>> it.  I've scrutinized the concurrency related aspects and still can't find
>> any bugs there.  However, I can't prove it's not a concurrency bug since
>> running in single threaded mode prevents certain code paths from being
>> taken.  Unless I get some advice that changes things, I think my next move
>> is to compare the disassemblies for cases that work to those for cases that
>> don't.
>>
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