[PATCH v4 7/7] configfs: Fix LOCKDEP nesting issues with fragment semaphores

[Mike Leach]

Hi Christoph

On Thu, 11 Aug 2022 at 16:09, Christoph Hellwig <hch at lst.de> wrote:
>
> On Fri, Aug 05, 2022 at 03:15:53PM +0100, Mike Leach wrote:
> > You are correct - in this patchset we are adding the use of a binary
> > attribute to load and unload Coresight configurations and features -
> > which action has a side effect of adding and removing entries in
> > particular sub-directories in our configfs sub-system.
>
> I don't think configfs was designed to be that dynamic.  Especially
> the fact that a write to a binary attribute, which is intended to
> be just for passthrough to hardware is a bit concerning.
>

configfs may not  have originally been intended to be used in quite
this way, but the system is robust and well designed, and the APIs
work perfectly well in the this way.
The model of user programming a configuration item for kernel backed
elements that have a userspace lifetime (as described in configfs.rst)
is precisely the model we need to successfully leverage the coresight
subsystem. Our requirements are more complex than the ACPI tables or
USB gadget drivers that use configfs, as we are potentially
configuring multiple coresight devices. What the user sees in the
configfs directories for the configurations are only the user
adjustable features of these configurations. The rest of the
programming is set on the relevant devices.

> From your further description it sounds like this binary attribute
> is loading structured data interpreted by the kernel, which really
> isn't how binary attributes in configfs or sysfs are intended to
> be used.

If you look at the ACPI configfs, then this is in fact loading an ACPI
table structure into the kernel, through a binary attribute, that is
validated and added to a list of tables, if it passes the validation.
This is also an example of using a binary attribute to pass structured
data into the kernel for subsequent interpretation - and this example
is why I chose to use binary attributes in this way.

The data passed through the coresight 'load' binary attribute is
validated, and then only if valid loaded into the coresight subsystem
for use when coresight trace is activated and the user decides to use
the configuration. (coresight configurations are loaded, but are only
used if specifically selected during a trace session).

Regards

Mike


-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK