In light of this new evidence, I think we don't need the full memory barrier. Please insert asm volatile("": : :"memory") before line 66 and after line 90 in XrdClMessageUtils.hh. If this helps, I will create a nicer patch for the built-in semaphores.
The semaphores are better here because we just need a very trivial synchronization mechanism. Posting/waiting on a semaphore involves just one atomic update and one syscall in case of congestion. Condvars are far more complex and, typically, involve multiple locks and multiple updates.
You would typically use a condvar if you have:
1. a complex condition for signaling
2. need to bump the owning thread's priority either because the critical section (the mutex) has a priority ceiling or because you're blocking a thread of higher priority and the mutex is in the inheritance mode.
If you're interested in how a condvar may be implemented, take a look at this: tb-condvar.c. I was once bored and implemented pretty much all of the pthread functions NPTL-style mostly in simple C: https://github.com/ljanyst/thread-bites
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