On Thu, Sep 12, 2013 at 05:58:49PM +0200, Daniel Vetter wrote: > On Thu, Sep 12, 2013 at 5:43 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > >> The one in ttm is just bonghits to shut up lockdep: ttm can recurse > >> into it's own pagefault handler and then deadlock, the trylock just > >> keeps lockdep quiet. We've had that bug arise in drm/i915 due to some > >> fun userspace did and now have testcases for them. The right solution > >> to fix this is to use copy_to|from_user_atomic in ttm everywhere it > >> holds locks and have slowpaths which drops locks, copies stuff into a > >> temp allocation and then continues. At least that's how we've fixed > >> all those inversions in i915-gem. I'm not volunteering to fix this ;-) > > > > Yikes.. so how common is it? If I simply rip the set_need_resched() out > > it will 'spin' on the fault a little longer until a 'natural' preemption > > point -- if such a thing is every going to happen. > > It's a case of "our userspace doesn't do this", so as long as you're > not evil and frob the drm device nodes of ttm drivers directly the > deadlock will never happen. No idea how much contention actually > happens on e.g. shared buffer objects - in i915 we have just one lock > and so suffer quite a bit more from contention. So no idea how much > removing the yield would hurt. If 'sane' userspace is never supposed to do this, then only insane userspace is going to hurt from this and that's a GOOD (tm) thing, right? ;-) And it won't actually deadlock if you don't use FIFO, for the regular scheduler class it'll just spin a little longer before getting preempted so no real worries there. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
