Re: [PATCH v5 0/3] rust: add `ww_mutex` support

["Onur Özkan" <work@onurozkan.dev>]

On Thu, 14 Aug 2025 15:22:57 -0300
Daniel Almeida <daniel.almeida@collabora.com> wrote:

> 
> Hi Onur,
> 
> > On 14 Aug 2025, at 12:56, Onur <work@onurozkan.dev> wrote:
> > 
> > On Thu, 14 Aug 2025 09:38:38 -0300
> > Daniel Almeida <daniel.almeida@collabora.com> wrote:
> > 
> >> Hi Onur,
> >> 
> >>> On 14 Aug 2025, at 08:13, Onur Özkan <work@onurozkan.dev> wrote:
> >>> 
> >>> Hi all,
> >>> 
> >>> I have been brainstorming on the auto-unlocking (on dynamic number
> >>> of mutexes) idea we have been discussing for some time.
> >>> 
> >>> There is a challange with how we handle lock guards and my current
> >>> thought is to remove direct data dereferencing from guards.
> >>> Instead, data access would only be possible through a fallible
> >>> method (e.g., `try_get`). If the guard is no longer valid, this
> >>> method would fail to not allow data-accessing after auto-unlock.
> >>> 
> >>> In practice, it would work like this:
> >>> 
> >>> let a_guard = ctx.lock(mutex_a)?;
> >>> let b_guard = ctx.lock(mutex_b)?;
> >>> 
> >>> // Suppose user tries to lock `mutex_c` without aborting the
> >>> // entire function (for some reason). This means that even on
> >>> // failure, `a_guard` and `b_guard` will still be accessible.
> >>> if let Ok(c_guard) = ctx.lock(mutex_c) {
> >>>    // ...some logic
> >>> }
> >>> 
> >>> let a_data = a_guard.try_get()?;
> >>> let b_data = b_guard.try_get()?;
> >> 
> >> Can you add more code here? How is this going to look like with the
> >> two closures we’ve been discussing?
> > 
> > Didn't we said that tuple-based closures are not sufficient when
> > dealing with a dynamic number of locks (ref [1]) and ww_mutex is
> > mostly used with dynamic locks? I thought implementing that
> > approach is not worth it (at least for now) because of that.
> > 
> > [1]:
> > https://lore.kernel.org/all/DBS8REY5E82S.3937FAHS25ANA@kernel.org
> > 
> > Regards,
> > Onur
> 
> 
> 
> I am referring to this [0]. See the discussion and itemized list at
> the end.
> 
> To recap, I am proposing a separate type that is similar to drm_exec,
> and that implements this:
> 
> ```
> a) run a user closure where the user can indicate which ww_mutexes
> they want to lock b) keep track of the objects above
> c) keep track of whether a contention happened
> d) rollback if a contention happened, releasing all locks
> e) rerun the user closure from a clean slate after rolling back
> f) run a separate user closure whenever we know that all objects have
> been locked. ```
> 
> In other words, we need to run a closure to let the user implement a
> given locking strategy, and then one closure that runs when the user
> signals that there are no more locks to take.
> 
> What I said is different from what Benno suggested here:
> 
> >>>>>>    let (a, c, d) = ctx.begin()
> >>>>>>        .lock(a)
> >>>>>>        .lock(b)
> >>>>>>        .lock(c)
> >>>>>>        .custom(|(a, _, c)| (a, c))
> >>>>>>        .lock(d)
> >>>>>>        .finish();
> 
> i.e.: here is a brief example of how the API should be used by
> clients:
> 
> ```
> // The Context keeps track of which locks were successfully taken.
> let locking_algorithm = |ctx: &Context| {
>   // client-specific code, likely some loop trying to acquire
> multiple locks: //
>   // note that it does not _have_ to be a loop, though. It up to the
> clients to // provide a suitable implementation here.
>   for (..) {
>     ctx.lock(foo); // If this succeeds, the context will add  "foo"
> to the list of taken locks. }
> 
>   // if this closure returns EDEADLK, then our abstraction must
> rollback, and // run it again.
> };
> 
> // This runs when the closure above has indicated that there are no
> more locks // to take.
> let on_all_locks_taken = |ctx: &Context| {
>   // everything is locked here, give access to the data in the guards.
> };
> 
> ctx.lock_all(locking_algorithm, on_all_locks_taken)?;
> ```
> 
> Yes, this will allocate but that is fine because drm_exec allocates
> as well.
> 
> We might be able to give more control of when the allocation happens
> if the number of locks is known in advance, e.g.:
> 
> ```
> struct Context<T> {
>   taken_locks: KVec<Guard<T>>,
> }
> 
> impl<T> Context<T> {
>   fn prealloc_slots(num_slots: usize, flags: ...) -> Result<Self> {
>     let taken_locks = ... // pre-alloc a KVec here. 
>     Self {
>       taken_slots,
>     } 
>   }
> }
> ```
> 
> The main point is that this API is optional. It builds a lot of
> convenience of top of the Rust WWMutex abstraction, but no one is
> forced to use it.
> 
> IOW: What I said should be implementable with a dynamic number of
> locks. Please let me know if I did not explain this very well. 
> 
> [0]:
> https://lore.kernel.org/rust-for-linux/8B1FB608-7D43-4DD9-8737-DCE59ED74CCA@collabora.com/

Hi Daniel,

Thank you for repointing it, I must have missed your previour mail.

It seems crystal clear, I will review this mail in detail when I am
working on this patch again.

Regards,
Onur