Re: [PATCH v2 7/7] rust: drm: make Driver::File lifetime-parameterized

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

On Sat Nov 7, 2026 at 9:16 AM CET, Alice Ryhl wrote:
> On Sat, Nov 07, 2026 at 03:03:25AM +0100, Danilo Krummrich wrote:
>> +        // Run driver destructors without `filelist_mutex`. Take each link before dropping its
>> +        // allocation to avoid recursively dropping the entire chain.
>> +        while let Some(mut data) = files {
>> +            files = data.as_mut().project().next.take();
>> +            drop(data);
>> +            drm.open_count.fetch_sub(1, Release);
>> +        }
>
> Instead of transferring all the files to another linked list, how about
> taking them off the file list one-by-one? If you release the mutex in
> between dropping each file, then we still avoid deadlocks if the file
> private data destructor takes the lock.

I did consider this, and technically it would (currently) work with
list_del_init(), but I did not like it because it would be a very subtle change
in semantics from the perspective of the C code. (I.e. for C drivers the list
entry would remain intact after this callback, whereas for Rust drivers it would
be removed.)

Actually, I did hesitate to move drop() out of the filelist_mutex protected path
at all because I don't think there's a way this could actually ever produce a
lock inversion, but the "move to a separate list" approach was pretty simple and
very few code so I thought it's worth ruling it out anyway.

(In any case, I'll soon start working on the C code to implement a common
lifecycle scheme.  Once we have that, the Rust code becomes even more straight
foward.)