Re: What does interruptibility mean exactly?

On Tue, Jun 15, 2010 at 12:41 PM, Simon Marlow wrote:

On 15/06/2010 09:00, Bas van Dijk wrote:

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On Mon, Jun 14, 2010 at 11:20 PM, Don Stewart  wrote:

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v.dijk.bas:

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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 enclosing block, BUT ONLY WHILE THE RESOURCE IS UNAVAILABLE"

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.

Thanks for the heads-up. BTW I just released threads-0.3 which, among other things, uses MVars again for waiting for the termination of a thread: http://hackage.haskell.org/package/threads-0.3 Regards, Bas