Combine ib_mutex_t::waiters and ib_mutex_t::lock_word

lock_word is boolean: 0 - mutex unlocked, 1 - mutex_locked
waiters is boolean: 0 - no waiters, 1 - there are suspended waiters

There are a few problems with maintaining these variables:

• we need to issue heavyweight memory barriers to ensure proper StoreLoad order
• even with memory barriers it didn't seem to work properly: we have utility thread that awakes stale waiters
• we need to load/store 2 variables instead of just 1, which may increase bus traffic

One of options would be to encode waiters into lock_word:
0 - unlocked
1 - locked, no waiters
>1 - locked, there are waiters

Ideally mutex_enter() should create just one acquire memory barrier, mutex_exit() should create just one release memory barrier.

One of possible options (neither tested nor benchmarked):

mutex_enter()

{

  for (;;)

  {

    for (i= 0; i < SPIN_ROUNDS; i++)

    {

      expected= 0;

      if (__atomic_compare_exchange_n(&lock_word, &expected, 1, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))

        return;

      delay();

    }

    if (__atomic_fetch_add(&lock_word, 1, __ATOMIC_ACQUIRE))

      suspend();

    else

      return;

  }

}

mutex_exit()

{

  if (__atomic_exchange_n(&lock_word, 0, __ATOMIC_RELEASE) > 1)

    wakeup_waiters();

}