On 15/06/2010 09:00, Bas van Dijk wrote:
On Mon, Jun 14, 2010 at 11:20 PM, Don Stewart<d...@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
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.
Cheers,
Simon
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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