Question about dec_lock_cnt method in Context class

Yuxin Ren ryx at gwmail.gwu.edu
Fri Aug 29 19:46:38 CEST 2014


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 at 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
>
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