Re: [PATCH v4.4.y] drivers: core: Remove glue dirs from sysfs earlier

[Sasha Levin <sashal@kernel.org>]

On Mon, Jan 28, 2019 at 09:31:30AM -0800, Zubin Mithra wrote:
>From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>
>commit 726e41097920a73e4c7c33385dcc0debb1281e18 upstream
>
>For devices with a class, we create a "glue" directory between
>the parent device and the new device with the class name.
>
>This directory is never "explicitely" removed when empty however,
>this is left to the implicit sysfs removal done by kobject_release()
>when the object loses its last reference via kobject_put().
>
>This is problematic because as long as it's not been removed from
>sysfs, it is still present in the class kset and in sysfs directory
>structure.
>
>The presence in the class kset exposes a use after free bug fixed
>by the previous patch, but the presence in sysfs means that until
>the kobject is released, which can take a while (especially with
>kobject debugging), any attempt at re-creating such as binding a
>new device for that class/parent pair, will result in a sysfs
>duplicate file name error.
>
>This fixes it by instead doing an explicit kobject_del() when
>the glue dir is empty, by keeping track of the number of
>child devices of the gluedir.
>
>This is made easy by the fact that all glue dir operations are
>done with a global mutex, and there's already a function
>(cleanup_glue_dir) called in all the right places taking that
>mutex that can be enhanced for this. It appears that this was
>in fact the intent of the function, but the implementation was
>wrong.
>
>Backport Note: kref_read() is not present in 4.4. Hence,
>use atomic_read(&kref.refcount) instead of kref_read(&kref).
>
>Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
>Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>Signed-off-by: Zubin Mithra <zsm@chromium.org>

Queued for 4.4, thank you.

--
Thanks,
Sasha