[PATCH] drm/nouveau/fbcon: fix deadlock with FBIOPUT_CON2FBMAP

[Daniel Vetter]

On Wed, Jul 13, 2016 at 02:40:50PM +0200, Peter Wu wrote:
> On Wed, Jul 13, 2016 at 11:54:49AM +0200, Daniel Vetter wrote:
> > On Tue, Jul 12, 2016 at 06:49:34PM +0200, Peter Wu wrote:
> > > The FBIOPUT_CON2FBMAP ioctl takes a console_lock(). When this is called
> > > while nouveau was runtime suspended, a deadlock would occur due to
> > > nouveau_fbcon_set_suspend also trying to obtain console_lock().
> > > 
> > > Fix this by delaying the drm_fb_helper_set_suspend call. Based on the
> > > i915 code (which was done for performance reasons though).
> > > 
> > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > > Signed-off-by: Peter Wu <peter at lekensteyn.nl>
> > > ---
> > > Tested on top of v4.7-rc5, the deadlock is gone.
> > 
> > If we bother with this, it should imo be moved into the drm_fb_helper.c
> > function drm_fb_helper_set_suspend(). But this also smells like some kind
> > of bad duct-tape. I think Lukas is working on some other rpm vs. fbdev
> > deadlocks, maybe we could fix them all with one proper fix? I've made some
> > comments on Lukas' last patch series.
> 
> This patch is only needed for drivers that use console_lock (for
> drm_fb_helper_set_suspend) in their runtime resume functions.
> Lukas posted fixes for runtime PM reference leaks, those are different
> from this deadlock (see
> https://lists.freedesktop.org/archives/dri-devel/2016-July/113005.html
> for a backtrace for this issue).
> 
> The deadlock could also be avoided if the device backing the fbcon is
> somehow runtime-resumed outside the lock, but that feels like a larger
> hack that does not seem easy.
> 
> The i915 patch was done to reduce resume time (due to console_lock
> contention), that feature seems useful to all other drivers too even if
> the deadlock is fixed in a different way.

I might have imagined something, but I thought Lukas is already working on
some rpm vs. vga_switcheroo inversions. But looking again this was on the
audio side.

I think the proper solution for fbcon would be for the fbdev emulation to
also hold a runtime pm references when the console is in use. This should
already happen correctly for vblank, the more tricky part is fbdev mmap
and fbcon:

- I have no idea, but there should be a way to intercept fbdev userspace
  mmaps and make sure that as long as an app has the fbdev mmapped we
  don't runtime suspend. No one really should be doing that (at least for
  normal setups), hence this shouldn't result in unsafe.

- For fbcon, we could suspend it in the dpms off callbacks (maybe with a
  timer), and resume it only when enabling fbcon again - fbcon needs to
  redraw anyway on dpms on.

  Another solution for fbcon might be to untangle the suspend/resume stuff
  and protect it by something else than console_lock. But that means
  fixing up fbcon locking horror shows.

> My current plan is to move stuff out of the lock and allow (just)
> resuming the console to be delayed.  Some drivers (nouveau,
> radeon/amdgpu, i915) do unnecessary stuff under the console lock:
> 
>  - nouveau: I *think* that cleraing/setting FBINFO_HWACCEL_DISABLED
>    (nouveau_fbcon_accel_restore) is safe outside the lock as the fb is
>    already suspended before clearing/after setting the flag.
>  - radeon: since the console is suspended, I don't think that that all
>    of the code is radeon_resume_kms is really needed.
>  - amdgpu: same as radeon. Btw, console_lock is leaked on an error path.
>  - i915: I think that clearing the fb memory can be done outside the
>    lock too as the console is suspended.
> 
> Please correct me if my assumptions are flawed.

Yeah, fixing this independent issues should definitely help, irrespective
of how we fix fb_set_suspend.

> > Besides this, when fixing a deadlock pls provide more details about the
> > precise callchain and the locks involved in the deadlock. If you
> > discovered this using lockdep, then just add the entire lockdep splat to
> > the commit message. Otherwise there's lots of guesswork involved here.
> > -Daniel
> 
> There was no lockdep splat, it was triggered via the ioctl in the commit
> message. I'll include the verbose trace from the previous mail in the
> next proposed patch to reduce hunting though.

Sounds good too.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch