BlockedIndefinitelyOnMVar exception

Neil Mitchell ndmitchell at
Sun Jul 4 05:30:19 EDT 2010

Hi Simon,

>> My suspicion for the root cause of the problem is that Concurrent.Chan
>> is incorrect. In the course of debugging this problem we found 2 bugs
>> in Chan, and while I never tracked down any other bugs in Chan, I no
>> longer trust it. By rewriting parts of the program, including avoiding
>> Chan, the bugs disappeared.I don't think I'll be using Chan again
>> until after someone has proven in correct.
> Considering Chan is <150 lines of code and has been around for many years,
> that's amazing!  Did you report the bugs?  Is it anything to do with
> asynchronous exceptions?

Nothing to do with async exceptions. I found:

Of course, there's also the async exceptions bug still around:

However, even after having a program with no async exceptions (I never
used them), and eliminating unGetChan and isEmpyChan, I still got
bugs. I have no proof they came from the Chan module, and no minimal
test case was ever able to recreate them, but the same program with my
own Chan implementation worked. My Chan had different properties (it
queues items randomly) and a subset of the Chan functions, so it still
doesn't prove any issue with Chan - but I am now sceptical.

> You should have more luck with Control.Concurrent.STM.TChan, incedentally.
>  It's much easier to get right, and when we benchmarked it, performance was
> about the same (all those withMVar/modifyMVars in Chan are quite expensive),
> plus you get to compose it easily: reading from either of 2 TChans is
> trivial.

The performance of the Haskell is irrelevant - the program spends all
its time invoking system calls. Looking at the implementation I am
indeed much more trusting of TChan, I'll be using that in future if
there is ever a need.

Thanks, Neil

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