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