Re: [Outreachy kernel] [PATCH] drm/amdgpu: use DEFINE_DEBUGFS_ATTRIBUTE with debugfs_create_file_unsafe()

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 30, 2020 at 09:25:18AM +0100, Greg KH wrote:
> On Fri, Oct 30, 2020 at 09:00:04AM +0100, Christian König wrote:
> > Am 30.10.20 um 08:57 schrieb Deepak R Varma:
> > > On Fri, Oct 30, 2020 at 08:11:20AM +0100, Greg KH wrote:
> > > > On Fri, Oct 30, 2020 at 08:52:45AM +0530, Deepak R Varma wrote:
> > > > > Using DEFINE_DEBUGFS_ATTRIBUTE macro with debugfs_create_file_unsafe()
> > > > > function in place of the debugfs_create_file() function will make the
> > > > > file operation struct "reset" aware of the file's lifetime. Additional
> > > > > details here: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.archive.carbon60.com%2Flinux%2Fkernel%2F2369498&data=04%7C01%7Cchristian.koenig%40amd.com%7Cddd7a6ac8164415a639708d87ca97004%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637396414464384011%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=o6GOHvMxNMuOPlC4nhDyURCHBLqfQZhYQq%2BiIMt3D3s%3D&reserved=0
> > > > > 
> > > > > Issue reported by Coccinelle script:
> > > > > scripts/coccinelle/api/debugfs/debugfs_simple_attr.cocci
> > > > > 
> > > > > Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
> > > > > ---
> > > > > Please Note: This is a Outreachy project task patch.
> > > > > 
> > > > >   drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 20 ++++++++++----------
> > > > >   1 file changed, 10 insertions(+), 10 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> > > > > index 2d125b8b15ee..f076b1ba7319 100644
> > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c
> > > > > @@ -1551,29 +1551,29 @@ static int amdgpu_debugfs_sclk_set(void *data, u64 val)
> > > > >   	return 0;
> > > > >   }
> > > > > -DEFINE_SIMPLE_ATTRIBUTE(fops_ib_preempt, NULL,
> > > > > -			amdgpu_debugfs_ib_preempt, "%llu\n");
> > > > > +DEFINE_DEBUGFS_ATTRIBUTE(fops_ib_preempt, NULL,
> > > > > +			 amdgpu_debugfs_ib_preempt, "%llu\n");
> > > > Are you sure this is ok?  Do these devices need this additional
> > > > "protection"?  Do they have the problem that these macros were written
> > > > for?
> > > > 
> > > > Same for the other patches you just submitted here, I think you need to
> > > > somehow "prove" that these changes are necessary, checkpatch isn't able
> > > > to determine this all the time.
> > > Hi Greg,
> > > Based on my understanding, the current function debugfs_create_file()
> > > adds an overhead of lifetime managing proxy for such fop structs. This
> > > should be applicable to these set of drivers as well. Hence I think this
> > > change will be useful.
> > 
> > Well since this is only created once per device instance I don't really care
> > about this little overhead.
> > 
> > But what exactly is debugfs doing or not doing here?
> 
> It is trying to save drivers from having debugfs files open that point
> to memory that can go away at any time.  For graphics devices, I doubt
> that is the case.

So for anything we add/remove we have two-stage cleanup

1. drm_dev_unregister (or drm_connector_unregisters, those are the only
two hotunpluggable things we have) unregister all the uapi interfaces.
This deletes all the sysfs and debugfs files.

2. Once all the references to the underlying object disappear from the
kernel, we free up the data structure.

Now for chardev and similar uapi, we hold full references for open files.
But for sysfs and debugfs we assume that those uapi layers will make sure
that after we deleted the files in step 1 all access through these
functions are guaranteed to have finished. If that's not the case, then we
need to rework our refcounting and also refcount the underlying drm
structure (drm_device or drm_connector) from sysfs/debugfs files.

Now I tried to look at the patch Deepak references, and I'm not really
clear what changes. Or whether we made a wrong assumption previously about
what debugfs/sysfs guarantee when we delete the files.
-Daniel

> 
> thanks,
> 
> greg k-h

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