mask, catch, myThreadId, throwTo

[Felipe Almeida Lessa]

Nice, thanks, Edward =).

On Tue, Apr 16, 2013 at 6:20 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> OK, I've updated the docus.
>
> Excerpts from Felipe Almeida Lessa's message of Mon Apr 15 13:34:50 -0700 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, =)
>>
>> [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html#g:14
>>
>> On Mon, Apr 15, 2013 at 5:25 PM, Bertram Felgenhauer
>> <bertram.felgenhauer at googlemail.com> 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.
>> >
>> >   http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html#v:throwTo
>> >
>> > 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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>>



-- 
Felipe.