Hi All,

I have some further understanding about the code.
If thread A can go back to its home cpu(core 1), this means it is not running on the core 2. 
And when A release its lock, it will switch to its helper.
During the context switch from thread A to its helper on core 2, the "_running_under_lock" 
of thread A should be set to false. Thus thread A is able to continue to run on its home cpu.

I think this logic has no problem. But I observed a thread going into the infinite loop in Switch_lock::set_lock_owner method.
in my test program.
Because of inappropriate memory barrier, or anything else?

Thank you very much.
Best,
Yuxin




On Wed, Aug 27, 2014 at 2:28 PM, Yuxin Ren <ryx@gwmail.gwu.edu> wrote:
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