[pflanze]

Regarding the pthread_mutex_lock code you posted: I'd say this shows that in the non-contention case there is no kernel call involved, assuming the used mutexes are of kind PTHREAD_MUTEX_NORMAL; atomic_exchange is likely entirely in user space[1] and thus only if the mutex is already locked will pthread_mutex_lock reach "wait" (wait_event ?) which is probably the kernel call; i.e. those are futexes.

Although I don't understand how the memory management makes use of mutexes (once a (any?) thread realizes that the GC needs to be run, how does it wait until all threads have reached a safe point?), and I don't have time to check ATM.

[1] http://www.insomniacgames.com/tech/articles/0807/files/multi... mentions the pthread_mutex_lock and unlock code including an implementation of atomic_exchange