Exclude Giant from adaptive locking.

[• hselasky]

Summary

Giant may be frequently locked over longer periods of time and is not fit
for adaptive mutex locking.

MFC after: 1 week
Sponsored by: NVIDIA Networking

[mjg]

I would say that's very unclear.

Can you point at the problematic Giant consumer? More to the point, is there a serious problem moving them off to a dedicated lock?

[• hselasky]

Add missing case.

[• hselasky]

@mjg:

It is a problem at least when loading and unloading PCI drivers, and during boot.

The problem is that syscons has a timer to disable the blank screen, which is Giant protected. This single Giant locked callout ends up blocking all callouts on the particular CPU it is invoked.

Also when Giant is locked for a longer timer all consumers like USB enumeration threads start spinning for nothing!

--HPS

[mav]

I think this should better be postponed a bit until we fix all other Giant consumers except newbus, assuming it is what causes the longest delays.