What does interruptibility mean exactly?

Simon Marlow marlowsd at gmail.com
Tue Jun 15 06:41:51 EDT 2010

On 15/06/2010 09:00, Bas van Dijk wrote:
> On Mon, Jun 14, 2010 at 11:20 PM, Don Stewart<dons at galois.com>  wrote:
>> v.dijk.bas:
>>> Hello,
>>> I've a short question about interruptible operations. In the following
>>> program is it possible for 'putMVar' to re-throw asynchronous
>>> exceptions even when asynchronous exception are blocked/masked?
>>>    newEmptyMVar>>= \mv ->  block $ putMVar mv x
>>> The documentation in Control.Exception about interruptible
>>> operations[1] confused me:
>>> "Some operations are interruptible, which means that they can receive
>>> asynchronous exceptions even in the scope of a block. Any function
>>> which may itself block is defined as interruptible..."
>> I think the best definition of interruptible is in this paper:
>>     www.haskell.org/~simonmar/papers/async.pdf
>> Section 5.3
> Thanks for the link Don! Next time I will re-read the paper before asking ;-)
> The definition makes it clear indeed:
> "Any operation which may need to wait indefinitely for a resource
> (e.g., takeMVar) may receive asynchronous exceptions even within an
> So I guess I can update my threads package to use MVars again. Nice!
> because they were a bit faster in an informal benchmark I performed
> some time ago.

This is currently true for takeMVar/putMVar but it is no longer true for 
throwTo (in 6.14+).  I'll update the docs to make that clear.  The 
reason is that throwTo now works by message passing when the target is 
on another CPU, and we consider a thread that is waiting for a response 
to a message to be blocked - even if the throwTo can in fact proceed 
immediately because the target is interruptible.


> A later quote from 5.3 emphasizes the definition even more:
> "...an interruptible operation cannot be interrupted if the resource
> it is attempting to acquire is always available..."
> The following darcs patch makes the definition of
> interruptibility in the documentation in Control.Exception a bit
> clearer in this regard:
> http://bifunctor.homelinux.net/~bas/doc-interruptibility.dpatch
> Regards,
> Bas
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