On 4 August 2014 17:55, Prarit Bhargava [off-list ref] wrote:

The issue is the collision between the setup & teardown of the policy's governor
sysfs files.

On creation the kernel does:

down_write(&policy->rwsem)
mutex_lock(kernfs_mutex) <- note this is similar to the "old" sysfs_mutex.

The opposite happens on a governor switch, specifically the existing governor's
exit, and then we get a lockdep warning.

Okay, probably a bit more clarity is what I was looking for. Suppose we try
to change governor, now tell me what will happen.

I tried to reproduce with the instructions but was unable to ... ut that was on
Friday ;) and I'm going to try again this morning.  I've also ping'd some of the
engineers here in the office who are working on ARM to get access to a system to
do further analysis and testing.

You DON'T need an ARM for that, just try that on any x86 machine which has
multiple groups of CPUs sharing clock line. Or in other terms there are multiple
policy structures there..

You just need to enable the flag we were discussing about, it just decided the
location where governor's directory will get created. Nothing else.