On 12/07, Srivatsa S. Bhat wrote:

CPU 0                         CPU 1
------                        ------

1. Acquire lock A             Increment CPU1's
                              atomic counter



2. Increment CPU0's            Try to acquire lock A
   atomic counter


Now consider what happens if a hotplug writer (cpu_down) begins,

Exactly. So the fake lockdep_map should be per-cpu as well.

lglock doesn't need this because lg_local_lock() is not recursive
and lockdep can catch the bug like this. So it can look as single
lock for lockdep.

IOW. If you use the global lockdep_map and want lockdep to notice
this deadlock you need rwlock_acquire_read(map, 0, 0, IP). But then
lockdep will complain if the task does "read lock" under "read lock".

Oleg.