mask, catch, myThreadId, throwTo

Felipe Almeida Lessa felipe.lessa at
Mon Apr 15 22:34:50 CEST 2013

Thanks a lot, you're correct!  The trouble is, I was misguided by the
"Interruptible operations" note [1] which states that

    The following operations are guaranteed not to be interruptible:
        ... * everything from Control.Exception ...

Well, it seems that not everything from Control.Exception fits the bill.

Thanks, =)


On Mon, Apr 15, 2013 at 5:25 PM, Bertram Felgenhauer
<bertram.felgenhauer at> wrote:
> Felipe Almeida Lessa wrote:
>> I have some code that is not behaving the way I thought it should.
>> The gist of it is
>>   sleeper =
>>     mask_ $
>>     forkIOWithUnmask $ \restore ->
>>       forever $
>>         restore sleep `catch` throwBack
>>   throwBack (Ping tid) = myThreadId >>= throwTo tid . Pong
>>   throwBack (Pong tid) = myThreadId >>= throwTo tid . Ping
>> Since (a) throwBack is executed on a masked state, (b) myThreadId is
>> uninterruptible, and (c) throwTo is uninterruptible, my understanding
>> is that the sleeper thread should catch all PingPong exceptions and
>> never let any one of them through.
> (c) is wrong, throwTo may block, and blocking operations are interruptible.
> explains this in some more detail.
> The simplest way that throwTo can actually block in your program, as
> far as I can see, and one that will only affect the threaded RTS, is
> if the sleeper thread and whichever thread is running the other
> throwBack are executing on different capabilities; this will always
> cause throwTo to block. (You could try looking at a ghc event log to
> find out more.)
> I last ran into trouble like that with System.Timeout.timeout; for
> that function I finally convinced myself that uninterruptibleMask
> is the only way to avoid such problems; then throwTo will not be
> interrupted by exceptions even when it blocks. Maybe this is the
> solution for your problem, too.
> Hope that helps,
> Bertram
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