Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups

From: J. Bruce Fields
Date: Thu Aug 09 2018 - 13:32:48 EST

I think there's also a problem with multiple tasks sharing the same
lock owner.

So, all locks are exclusive locks for the same range. We have four
tasks. Tasks 1 and 4 share the same owner, the others' owners are

- Task 1 gets a lock.
- Task 2 gets a conflicting lock.
- Task 3 gets another conflicting lock. So now we the tree is
- Task 1's lock is released.
- Before task 2 is scheduled, task 4 acquires a new lock.
- Task 2 waits on task 4's lock, we now have

Task 3 shouldn't be waiting--the lock it's requesting has the same owner
as the lock task 4 holds--but we fail to wake up task 3.