Question about dec_lock_cnt method in Context class
ryx at gwmail.gwu.edu
Wed Aug 27 20:28:05 CEST 2014
I have a question about the dec_lock_cnt method in Context class under
In its implementation, it checks if thread's home cpu is equal to current
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.
bool have_no_locks = o->_lock_cnt < 1;
assert_kdb (current_cpu() == o->home_cpu());
if (EXPECT_TRUE(mp_cas(&o->_running_under_lock, Mword(false),
Do I misunderstand anything here?
Thank you very much.
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