Created attachment 98395 [details]
dmesg

If GPU blacklist is disabled in Chromium (on chrome://flags; so hardware
accelerated rendering become enabled, judging from chrome://gpu) GPU sometimes
lockup on just opening new tab (at least one time per day). After lockup I
observe same behaviour like in Bug 77892 - reset happens but X is unresponsive
except mouse.

Radeon HD 6620G, Kubuntu 14.04 x86_64, Linux 3.15rc3, X.Org Server 1.15.1,
libdrm 2.4.54+git1405030630.5126fc, Mesa 10.3~git1405030730.64c467, radeon
driver 7.3.99+git1405030730.be1469.

As I remember, this behaviour (GPU hang on opening new tab) introduced since
Linux 3.15.