Created attachment 144489 [details]
full dmesg output with drm.debug=0xe

Since 5.1-rc1 mainline linux kernel release, webpage contents in firefox hang randomly.  Hangs are non-fatal and "fixable" with alt-tab keypress or an elevated background system load.

I have bisected this issue to the commit 754a254427:

commit 754a25442705c4f90e0d05f1a7bd303ffe700ca9
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Mar 21 16:19:08 2019 +0000

    drm/i915: Skip object locking around a no-op set-domain ioctl


I have verified that this issue still present in both current Linus' tree and drm-tip.  Reverting the commit fixes the problem.

Steps to reproduce:
1. Launch firefox (tested versions 66 and 67), open any long webpage with vertical scrollbar.
2. Scroll the page until it becomes stuck.

(hangs can also happen on pages without scrollbar, it is just easier to notice)

Problem is always reproducible after few attempts.

System information:
uname -m: x86_64
uname -r: 5.2.0-rc3-00948-g999689d568a8-dirty (aka current drm-tip with an unrelated local change)
system: Debian sid
hardware: Xiaomi mi notebook air 12"; Timi TM1612/TM1612, BIOS A04 08/06/2016
display connector: eDP
full dmesg is attached.