Re: [PATCH v5 5/7] revocable: Add fops replacement

["Danilo Krummrich" <dakr@kernel.org>]

On Fri Oct 17, 2025 at 8:44 PM CEST, Jason Gunthorpe wrote:
> On Fri, Oct 17, 2025 at 08:19:06PM +0200, Danilo Krummrich wrote:
>> On 10/17/25 6:37 PM, Jason Gunthorpe wrote:
>> > On Fri, Oct 17, 2025 at 06:29:10PM +0200, Danilo Krummrich wrote:
>> > 
>> >> I'm not sure about MISC device though. Unless there's a good reason,
>> >> I think MISC device should be "fenced" instead.
>> > 
>> > misc is a very small wrapper around raw fops, and raw fops are
>> > optimized for performance. Adding locking that many important things
>> > like normal files don't need to all fops would not be agreed.
>> > 
>> > The sketch in this series where we have a core helper to provide a
>> > shim fops that adds on the lock is smart and I think could be an
>> > agreeable way to make a synchronous misc and cdev unregister for
>> > everyone to trivially use.
>> 
>> Sure, for MISC devices without a parent for instance there are no device
>> resources to access anyways.
>
> There are many situations with misc that can get people into trouble without
> parent:
>
>  misc_deregister(x);
>  timer_shutdown_sync(y);
>  kfree(z);
>
> For example. It is is buggy if the fops touch y or z.
>
> This is why a _sync version is such a nice clean idea because with 5
> letters the above can just be fixed.
>
> Wrapping everything in a revocable would be a huge PITA.

That's a bit of a different problem though. Revocable clearly isn't the
solution. _sync() works, but doesn't account for the actual problem, which is
that the file private has at least shared ownership of y and z.

So, it's more of an ownership / lifetime problem. The file private data should
either own y and z entirely or a corresponding reference count that is dropped
in fops release().

Device resources are different though, since we can't just hold on to them with
a reference count etc.; they're strictly gone once the bus device is unbound,
hence revocable when there is no _sync().