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@infradead.org 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.