Currently, a single lock can only lock a single target for simple. Lock mode can not be dynamicly downgraded/upgraded, you can use another lock instead.
It's NOT required that lock/unlock in the same thread. But you should explicitly call unlock if you called lock, no matter successes or not. An unlocked MGLock will lead to crash on its destructor, you should wrap your own RAII class above MGL.
On each lock, two lists are maintained, the running-list and the conflict-list. If the incoming lock requirement is compitable with the running-list, the lock is granted immediatly, otherwise it is added into the conflict-list.
In order to avoid starvation, If the conflict list is not empty, new requests are added into conflict-list even if it's compitable with the running-list.
- deadlock detect