mask, catch, myThreadId, throwTo
Edward Z. Yang
ezyang at MIT.EDU
Tue Apr 16 23:20:56 CEST 2013
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  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, =)
>  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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