Re: [PATCH 1/2] drm: Add might_fault to drm_modeset_lock priming

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Wed, Jul 10, 2024 at 02:40:04PM +0200, Christian König wrote:
> Am 10.07.24 um 13:58 schrieb Daniel Vetter:
> > On Wed, 10 Jul 2024 at 13:39, Christian König <christian.koenig@amd.com> wrote:
> > > Am 10.07.24 um 11:31 schrieb Daniel Vetter:
> > > > We already teach lockdep that dma_resv nests within drm_modeset_lock,
> > > > but there's a lot more: All drm kms ioctl rely on being able to
> > > > put/get_user while holding modeset locks, so we really need a
> > > > might_fault in there too to complete the picture. Add it.
> > > Mhm, lockdep should be able to deduce that when there might be faults
> > > under the dma_resv lock there might also be faults under the
> > > drm_modeset_lock.
> > You're not allowed to take a fault under dma_resv, because drivers
> > might need to take that lock to handle faults. So unfortunately in our
> > combined lockdep priming, there really seems to be no chain yet that
> > teaches about faults possibly happening while holding
> > drm_modeset_lock.
> 
> Ah, of course! You are right, it was just the other way around.

Applied to drm-misc-next, thanks for your review.
-Sima

> 
> Thanks,
> Christian.
> 
> > -Sima
> > 
> > > > Motivated by a syzbot report that blew up on bcachefs doing an
> > > > unconditional console_lock way deep in the locking hierarchy, and
> > > > lockdep only noticing the depency loop in a drm ioctl instead of much
> > > > earlier. This annotation will make sure such issues have a much harder
> > > > time escaping.
> > > > 
> > > > References: https://lore.kernel.org/dri-devel/00000000000073db8b061cd43496@google.com/
> > > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > > Cc: Maxime Ripard <mripard@kernel.org>
> > > > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > > > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > > > Cc: "Christian König" <christian.koenig@amd.com>
> > > > Cc: linux-media@vger.kernel.org
> > > > Cc: linaro-mm-sig@lists.linaro.org
> > > On the other hand pointing it out explicitly doesn't hurts us at all, so
> > > Reviewed-by: Christian König <christian.koenig@amd.com>.
> > > 
> > > Regards,
> > > Christian.
> > > 
> > > > ---
> > > >    drivers/gpu/drm/drm_mode_config.c | 2 ++
> > > >    1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> > > > index 568972258222..37d2e0a4ef4b 100644
> > > > --- a/drivers/gpu/drm/drm_mode_config.c
> > > > +++ b/drivers/gpu/drm/drm_mode_config.c
> > > > @@ -456,6 +456,8 @@ int drmm_mode_config_init(struct drm_device *dev)
> > > >                if (ret == -EDEADLK)
> > > >                        ret = drm_modeset_backoff(&modeset_ctx);
> > > > 
> > > > +             might_fault();
> > > > +
> > > >                ww_acquire_init(&resv_ctx, &reservation_ww_class);
> > > >                ret = dma_resv_lock(&resv, &resv_ctx);
> > > >                if (ret == -EDEADLK)
> > 
> 

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