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

["Christian König" <christian.koenig@amd.com>]

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.

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)
>