On Thu, Nov 19, 2020 at 01:25:11PM +0000, Chris Wilson wrote: > Quoting Peter Zijlstra (2020-11-19 13:02:44) > > > > Chris, I suspect this is due to i915 calling stop machine with all sorts > > of locks held. Is there anything to be done about this? stop_machine() > > is really nasty to begin with. > > > > What problem is it typing to solve? > > If there is any concurrent access through a PCI bar (that is exported to > userspace via mmap) as the GTT is updated, results in undefined HW > behaviour (where that is not limited to users writing to other system > pages). > > stop_machine() is the most foolproof method we know that works.
Sorry, I don't understand. It tries to do what? And why does it need to do that holding locks. Really, this is very bad form. > This particular cycle is easy to break by moving the copy_to_user to > after releasing perf_event_ctx_unlock in perf_read(). The splat in question is about the ioctl()s, but yeah that too. Not sure how easy that is. I'm also not sure that'll solve your problem, cpu_hotplug_lock is a big lock, there's tons of stuff inside.
