[cpp-threads] Re: Exception propogation across threads

[Vijay Saraswat]

Boehm, Hans wrote:

>>Unwind the stack, terminate the thread and throw some kind of 
>>unexpected thread-death exception in the joining thread. Less 
>>severe than (i) and (ii), less compiler magic or modification 
>>of exceptions needed than
>>(iii) and (iv), and more usable than (v).
>>
>>    
>>
>My recollection from the last discussion is that this has the (probably
>fatal?) flaw that the join may not be executed until next month.  Thus
>you could effectively be hiding the exception for a month.  Since it
>escaped the thread, it was presumably not completely handled, so we are
>now letting the application run in a bad state for a month, potentially
>wreaking all sorts of havoc.
>
>  
>
A word about how we solved this in X10 (http://x10.sf.net) -- a language 
in the Java framework focused on constructs for concurrency and 
distribution.

We introduced a scoped join construct, finish S, where S is a statement. 
The thread executing finish S runs S, and then waits for all threads 
that were launched during the execution of S to terminate -- normally or 
abruptly (by throwing an exception). Once all threads have terminated, 
the set of exceptions (if any) thrown by the subservient threads is then 
encapsulated in a single exception thrown in the parent thread.

Thus there are no race conditions. This works extremely well, 
particularly for structured (nested) parallelism, as supported by Cilk 
and X10. One can start the main program in a finish. Now there is a 
thread at the very root of the "spawns" tree that can catch any cross 
thread exception.

More details on the rooted exception model at p 22 of 
http://x10.sourceforge.net/docs/x10-101.pdf

Best,
Vijay

-- 
X10: Programming parallel machines, productively (http://x10.sf.net)

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