unlock(rl::ReentrantLock) does a notify(cond) which wakes all tasks waiting on the lock. As this is a lock, only one of these tasks can succeed, so it is unnecessary to schedule all of them. When there are a large number of tasks, the current approach wastes CPU time and can cause a quadratic amount of allocations for waitlist nodes.

It isn't straightforward to trivially fix this by changing the notify call to set all=false -- unlock will only call notify if ReentrantLock.havelock is 0x02 (see here) and that only happens after the first notify if all waiting tasks are woken up to race for the lock again (see here).

Broken out from #50425 as this is a separate issue.