[Condition] Is it possible to make an await() WITHOUT releasing lock?

Question

Like title says...I expose my problem:
I need to impose a FIFO ordering between 2 kind of threads, A and B. A and B come together in the critical section, here A has to check that NO thread of type B is in the section and, if it is not, pass and go in another section and, at the end, modify a variable value in order for B to enter. However, more threads of type A can be together in the 2nd section.
So, if for example we have:
BAAA> | critical | 2nd section |
3 A can enter in 2nd section but 1 B stays outside the critical section.
critical section is like this with condition variables:
lock.lock();
               while(nThreadBInside > 0)
                    waitCond.await();problem is that await() awakens another thread in the queue, so suppose a situation like this:
BABA->
A enters.
B blocks on await unlocking the lock
A enters...no FIFO ordering!
Ho can I solve this?:(

Answer 1

Use a simple in memory flag-- b.inWait(), be awakens checks to see if the flag is still set and if it is goes back to a wait state--works for single events.
In the event you have multiple blockers, you need to keep count of the numer of blockers that have a hold on the wait state, so. Each time an A enters the critical, increment a counter. When b comes out of wait, then have it check the counter--Counter>0, then b goes back to wait. When each A leaves the critical segment, then decrrement the counter.