[boost] [Interprocess::Semaphore] Deadlock on more producers - one consumer problem

Subject: [boost] [Interprocess::Semaphore] Deadlock on more producers - one consumer problem
From: Cosimo Calabrese (cosimo.calabrese_at_[hidden])
Date: 2009-06-17 11:58:18

Hi to all,

I've a buffer of 10 positions; 4 producers put elements on the buffer 
and only 1 consumer pick them; the producers generate elements very 
fastly, but the consumer picks the element very slowly. I've decided to 
synchronize the producers and the consumer with a boost::interprocess 
semaphore initialized to 10.

Before to put an element in the buffer, the producer execute a wait() in 
the semaphore; the 4 producers put quickly 10 elements in the buffer, so 
they all block on the wait(). When the consumer picks an element from 
the buffer, it performs a post() on the semaphore.

Here is the problem: the post() unlocks only one of the 4 producers. 
Consider this scenario:

- the producers are all in wait state; the semaphore counter is 0;
- the consumer picks an element and perform a post(), that unlocked the 
producer_1; the semaphore counter is 1;
- the producer_1 is unlocked but it doesn't produce any element; the 
other producers are locked;
- the consumer picks the other 9 elements in the buffer and perform the 
9 post() on the semaphore, but the semaphore counter becomes 2, 3, 4, 
... 10, and so the semaphore doesn't signal the other producers, that 
remains in lock state(). --> DEADLOCK.

Is there a way to unlock all the producers with the 
boost::interprocess::semaphore? Or I have mistaken the approach of the 
problem?

Thanks to all,
Cosimo Calabrese.