Ronan Cunniffe wrote:
>> On Tue, 25 May 2004, Bryan O'Donoghue wrote:
>>>>>I'm not sure I understand: is the problem that thread 1 might update twice
>>>and at least one of threads 2 to n might miss a transition?
>>>>The problem is that thread 2: can send data before or after thread 1 has
>>executed. A race condition I guess you'd call it.
>>>>The pthread_cond_wait/broadcast logic assumes thread 1 to be in a wait state
>>for thread 2, to signal it and wake it up.
>>> Hmmm. You can avoid "assumptions" by getting each thread to write down
> (under mutex protection) what its state is. Note that pthread_cond_wait()
> explicitly allows you to atomically wait on a condition *and* release a
> mutex. In other words, the thread goes into a wait state from inside
> mutex protection.
>>>>>This is no good if thread 2 is finished it's execution before thread 1 has put
>>itself into a wait state.
>>> Here's one possible way round this (if I've understood correctly)
>> Thread 1:
>> a) lock mutex
> b) if thread 2 is marked as finished, release mutex and don't wait.
> c) otherwise, mark self as being in a wait state, and call
> pthread_cond_wait (releasing mutex at the same time).
>>> Thread 2: (before exiting)
> a) lock mutex
> b) if thread 1 is marked as waiting, fire pthread_cond_signal()
> c) mark self as finished
> d) release mutex.
>>> The most powerful ideas here (I think) are:
>> 1) Each thread records its own condition
> 2) Each thread decides (based on what it has just done and the condition
> of other threads) what threads need signalling.
> 3) pthread_cond_wait does 2 things: thread -> waiting; release mutex
Hmm.
I like it in that its faster and doesn't involve polling, the many state
variables scare me, there's easily six functions to synchronise like this,
across multiple threads.
Though so long as I don't get multiple threads affecting the same state
variables that should solve it.
--
Bryan O'Donoghue
Embedded Software Engineer
Europlex Technologies Ltd
Clonshaugh Business & Technology Park
Dublin 17
Ireland
T:+353 (0) 1 2500500
F:+353 (0) 1 2500590
E:bryano at euoplex.ie
W:www.europlex.ie
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