Linux pthread mutex and kernel scheduler

Question

With a friend of mine, we disagree on how synchronization is handled at userspace level (in the pthread library).

a. I think that during a pthread_mutex_lock, the th

Answer 1

Some debuging with gdb for this test program:

#include 
#include 
#include 
#include 

pthread_mutex_t x = PTHREAD_MUTEX_INITIALIZER;

void* thr_func(void *arg)
{
  pthread_mutex_lock(&x);
}

int main(int argc, char **argv)
{
  pthread_t thr;

  pthread_mutex_lock(&x);
  pthread_create(&thr, NULL, thr_func, NULL);
  pthread_join(thr,NULL);
  return 0;
}

shows that a call to pthread_mutex_lock on a mutex results in a calling a system call futex with the op parameter set to FUTEX_WAIT (http://man7.org/linux/man-pages/man2/futex.2.html)

And this is description of FUTEX_WAIT:

FUTEX_WAIT

This operation atomically verifies that the futex address uaddr still contains the value val, and sleeps awaiting FUTEX_WAKE on this futex address. If the timeout argument is non-NULL, its contents describe the maximum duration of the wait, which is infinite otherwise. The arguments uaddr2 and val3 are ignored.

So from this description I can say that if a mutex is locked then a thread will sleep and not actively wait. And it will sleep until futex with op equal to FUTEX_WAKE is called.