Question about dec_lock_cnt method in Context class

Yuxin Ren ryx at gwmail.gwu.edu
Wed Aug 27 20:28:05 CEST 2014


Hi All,

I have a question about the dec_lock_cnt method in Context class under
multiple processor.
In its implementation, it checks if thread's home cpu is equal to current
cpu.
If not, it does not unset "_running_under_lock" variable, even if the
_lock_cnt is 0.
Why does it check if home cpu is equal to current cpu?

Consider the following case:
Thread A is running on core 1 holding a helping lock, but is preempted by
another high priority thread.
Thread B is running on core 2, and tries to take the same helping lock. It
finds the lock is held by A, so
it helps A to run by migrating A to core 2.
When A releases its lock, finding the current cpu is not its home cpu, so
it does not set "_running_under_lock" to false.
Then A continues to run on core 1 and tries to take that lock again. But
now it can never take the lock, as the implementation
of helping lock.

PRIVATE inline
bool NO_INSTRUMENT
Switch_lock::set_lock_owner(Context *o)
{
  bool have_no_locks = o->_lock_cnt < 1;

  if (have_no_locks)
    {
      assert_kdb (current_cpu() == o->home_cpu());
      for (;;)
        {
          if (EXPECT_FALSE(access_once(&o->_running_under_lock)))
            continue;
          if (EXPECT_TRUE(mp_cas(&o->_running_under_lock, Mword(false),
Mword(true))))
            break;
        }
    }
    ...
}

Do I misunderstand anything here?

Thank you very much.
Best,
Yuxin
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