can you block a thread in GHC by its threadID?

Bernard James POPE bjpop at
Tue Jun 22 05:38:48 EDT 2004

On Tue, Jun 22, 2004 at 09:45:48AM +0100, Simon Marlow wrote:
> On 22 June 2004 06:11, Bernard James POPE wrote:
> > Ideally I'd like this function:
> > 
> >    blockThread :: ThreadId -> IO ()
> > 
> > and thus:
> > 
> >    unBlockThread :: ThreadId -> IO ()
> Hmm, might be possible.  Can the blocked thread be woken up by an
> exception?  (this is usually the case for blocked threads).

I guess so.

> Note that if you block a thread and then drop all references to it, the
> garbage collector will wake up the thread with a BlockedOnDeadMVar
> exception.

That sounds reasonable.

> I think I'd be tempted to call these functions {stop,continue}Thread to
> avoid overloading the block/unblock terms any more.  Stop/continue is
> used in Unix land too.

Yes, those names are fine (I was thinking of suspend/resume).

> To implement this you'll need another StgTSOBlockReason state for
> stopped threads.  Stopping already blocked threads might not be a
> problem, since (in some cases at least) the blocking operation will be
> retried when the thread is started again.  I'm not sure whether this is
> always the case though.  
> Stopping a thread blocked on a foreign call
> cannot be done.  Stopping a thread blocked on I/O or delay# will need to
> remove the thread from the appropriate queue.
> You'll need two new primops: stopThread#, continueThread#.  Take a look
> at the implementation of killThread# for clues (in
> ghc/rts/Exception.hc).  Don't forget to take into account the case when
> a thread stops itself (that's the tricky one).  

Hmm. In the worst case I can just ban it by comparing the
threadIds, and require the use of yield?

> Let us know if you need any more guidance...

Thanks. I'll start looking into it in more detail.

Supposing that such a thing is indeed possible is there any chance that
it could be folded into GHC? (Then I wouldn't have to ship my own variant
of the runtime with buddha.)


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