[PATCH v3 10/12] drm/amdgpu: Avoid sysfs dirs removal post device unplug

[Daniel Vetter]

On Tue, Nov 24, 2020 at 11:27 PM Andrey Grodzovsky
<Andrey.Grodzovsky at amd.com> wrote:
>
>
> On 11/24/20 9:49 AM, Daniel Vetter wrote:
> > On Sat, Nov 21, 2020 at 12:21:20AM -0500, Andrey Grodzovsky wrote:
> >> Avoids NULL ptr due to kobj->sd being unset on device removal.
> >>
> >> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
> >> ---
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c   | 4 +++-
> >>   drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c | 4 +++-
> >>   2 files changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> >> index caf828a..812e592 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> >> @@ -27,6 +27,7 @@
> >>   #include <linux/uaccess.h>
> >>   #include <linux/reboot.h>
> >>   #include <linux/syscalls.h>
> >> +#include <drm/drm_drv.h>
> >>
> >>   #include "amdgpu.h"
> >>   #include "amdgpu_ras.h"
> >> @@ -1043,7 +1044,8 @@ static int amdgpu_ras_sysfs_remove_feature_node(struct amdgpu_device *adev)
> >>              .attrs = attrs,
> >>      };
> >>
> >> -    sysfs_remove_group(&adev->dev->kobj, &group);
> >> +    if (!drm_dev_is_unplugged(&adev->ddev))
> >> +            sysfs_remove_group(&adev->dev->kobj, &group);
> > This looks wrong. sysfs, like any other interface, should be
> > unconditionally thrown out when we do the drm_dev_unregister. Whether
> > hotunplugged or not should matter at all. Either this isn't needed at all,
> > or something is wrong with the ordering here. But definitely fishy.
> > -Daniel
>
>
> So technically this is needed because kobejct's sysfs directory entry kobj->sd
> is set to NULL
> on device removal (from sysfs_remove_dir) but because we don't finalize the device
> until last reference to drm file is dropped (which can happen later) we end up
> calling sysfs_remove_file/dir after
> this pointer is NULL. sysfs_remove_file checks for NULL and aborts while
> sysfs_remove_dir
> is not and that why I guard against calls to sysfs_remove_dir.
> But indeed the whole approach in the driver is incorrect, as Greg pointed out -
> we should use
> default groups attributes instead of explicit calls to sysfs interface and this
> would save those troubles.
> But again. the issue here of scope of work, converting all of amdgpu to default
> groups attributes is somewhat
> lengthy process with extra testing as the entire driver is papered with sysfs
> references and seems to me more of a standalone
> cleanup, just like switching to devm_ and drmm_ work. To me at least it seems
> that it makes more sense
> to finalize and push the hot unplug patches so that this new functionality can
> be part of the driver sooner
> and then incrementally improve it by working on those other topics. Just as
> devm_/drmm_ I also added sysfs cleanup
> to my TODO list in the RFC patch.

Hm, whether you solve this with the default group stuff to
auto-remove, or remove explicitly at the right time doesn't matter
much. The underlying problem you have here is that it's done way too
late. sysfs removal (like all uapi interfaces) need to be removed as
part of drm_dev_unregister. I guess aside from the split into fini_hw
and fini_sw, you also need an unregister_late callback (like we have
already for drm_connector, so that e.g. backlight and similar stuff
can be unregistered).

Papering over the underlying bug like this doesn't really fix much,
the lifetimes are still wrong.
-Daniel

>
> Andrey
>
>
> >
> >>
> >>      return 0;
> >>   }
> >> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c
> >> index 2b7c90b..54331fc 100644
> >> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c
> >> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c
> >> @@ -24,6 +24,7 @@
> >>   #include <linux/firmware.h>
> >>   #include <linux/slab.h>
> >>   #include <linux/module.h>
> >> +#include <drm/drm_drv.h>
> >>
> >>   #include "amdgpu.h"
> >>   #include "amdgpu_ucode.h"
> >> @@ -464,7 +465,8 @@ int amdgpu_ucode_sysfs_init(struct amdgpu_device *adev)
> >>
> >>   void amdgpu_ucode_sysfs_fini(struct amdgpu_device *adev)
> >>   {
> >> -    sysfs_remove_group(&adev->dev->kobj, &fw_attr_group);
> >> +    if (!drm_dev_is_unplugged(&adev->ddev))
> >> +            sysfs_remove_group(&adev->dev->kobj, &fw_attr_group);
> >>   }
> >>
> >>   static int amdgpu_ucode_init_single_fw(struct amdgpu_device *adev,
> >> --
> >> 2.7.4
> >>



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