Re: [PATCH v7 5/6] rust: ww_mutex: implement LockSet

[Lyude Paul <lyude@redhat.com>]

On Mon, 2025-11-24 at 18:49 +0300, Onur Özkan wrote:
> > 
> > I wonder if there's some way we can get rid of the safety contract
> > here and verify this at compile time, it would be a shame if every
> > single lock invocation needed to be unsafe.
> > 
> 
> Yeah :(. We could get rid of them easily by keeping the class that was
> passed to the constructor functions but that becomes a problem for the
> from_raw implementations.
> 
> I think the best solution would be to expose ww_class type from
> ww_acquire_ctx and ww_mutex unconditionally (right now it depends on
> DEBUG_WW_MUTEXES). That way we can just access the class and verify
> that the mutex and acquire_ctx classes match.
> 
> What do you think? I can submit a patch for the C-side implementation.
> It should be straightforward and shouldn't have any runtime impact.

I would be fine with this, and think this is definitely the right way to go

> 
> > > +    ///
> > > +    ///         Ok(())
> > > +    ///     },
> > > +    ///     // `on_all_locks_taken` closure
> > > +    ///     |lock_set| {
> > > +    ///         // Safely mutate both values while holding the
> > > locks.
> > > +    ///         lock_set.with_locked(&mutex1, |v| *v += 1)?;
> > > +    ///         lock_set.with_locked(&mutex2, |v| *v += 1)?;
> > > +    ///
> > > +    ///         Ok(())
> > > +    ///     },
> > 
> > I'm still pretty confident we don't need or want both closures and
> > can combine them into a single closure. And I am still pretty sure
> > the only thing that needs to be tracked here is which lock we failed
> > to acquire in the event of a deadlock.
> > 
> > Let me see if I can do a better job of explaining why. Or, if I'm
> > actually wrong about this - maybe this will help you correct me and
> > see where I've misunderstood something :).
> > 
> > First, let's pretend we've made a couple of changes here:
> > 
> >   * We remove `taken: KVec<RawGuard>` and replace it with `failed: *mut
> >     Mutex<…>`
> >   * lock_set.lock():
> >      - Now returns a `Guard` that executes `ww_mutex_unlock` in its
> > destructor
> >      - If `ww_mutex_lock` fails due to -EDEADLK, this function stores
> > a pointer to the respective mutex in `lock_set.failed`.
> >      - Before acquiring a lock, we now check:
> >         + if lock_set.failed == lock
> >            * Return a Guard for lock without calling ww_mutex_lock()
> >            * lock_set.failed = null_mut();
> >   * We remove `on_all_locks_taken()`, and rename `locking_algorithm` to
> >     `ww_cb`.
> >   * If `ww_cb()` returns Err(EDEADLK):
> >      - if !lock_set.failed.is_null()
> >         + ww_mutex_lock(lock_set.failed) // Don't store a guard
> >   * If `ww_cb()` returns Ok(…):
> >      - if !lock_set.failed.is_null()
> >        // This could only happen if we hit -EDEADLK but then `ww_cb`
> > did not // re-acquire `lock_set.failed` on the next attempt
> >         + ww_mutex_unlock(lock_set.failed)
> > 
> > With all of those changes, we can rewrite `ww_cb` to look like this:
> > 
> > > lock_set| {
> > // SAFETY: Both `lock_set` and `mutex1` uses the same class.
> > let g1 = unsafe { lock_set.lock(&mutex1)? };
> > 
> > // SAFETY: Both `lock_set` and `mutex2` uses the same class.
> > let g2 = unsafe { lock_set.lock(&mutex2)? };
> > 
> > *g1 += 1;
> > *g2 += 2;
> > 
> > Ok(())
> > }
> > 
> > If we hit -EDEADLK when trying to acquire g2, this is more or less
> > what would happen:
> > 
> >   * let res = ww_cb():
> >      - let g1 = …; // (we acquire g1 successfully)
> >      - let g2 = …; // (enter .lock())
> >         + res = ww_mutex_lock(mutex2);
> >         + if (res) == EDEADLK
> >            * lock_set.failed = mutex2;
> >         + return Err(EDEADLK);
> >      - return Err(-EDEADLK);
> >        // Exiting ww_cb(), so rust will drop all variables in this
> > scope:
> >         + ww_mutex_unlock(mutex1) // g1's Drop
> > 
> >   * // (res == Err(EDEADLK))
> >     // All locks have been released at this point
> > 
> >   * if !lock_set.failed.is_null()
> >      - ww_mutex_lock(lock_set.failed) // Don't create a guard
> >     // We've now re-acquired the lock we dead-locked on
> > 
> >   * let res = ww_cb():
> >      - let g1 = …; // (we acquire g1 successfully)
> >      - let g2 = …; // (enter .lock())
> >         + if lock_set.failed == lock
> >            * lock_set.failed = null_mut();
> >            * return Guard(…); // but don't call ww_mutex_lock(), it's
> > already locked
> >      - // We acquired g2 successfully!
> >      - *g1 += 1;
> >      - *g2 += 2;
> > 
> >   * etc…
> > 
> > The only challenge with this is that users need to write their ww_cb()
> > implementations to be idempotent (so that calling it multiple times
> > isn't unexpected). But that's already what we do on the C side, and
> > is kind of what I expected we would want to do in rust anyhow.
> > 
> > Does this make sense, or was there something I made a mistake with
> > here?
> 
> Thanks a lot for analyzing and providing an alternative on this!
> 
> However, collapsing everything into a single callback would require the
> caller to self-police various disciplines like "don't touch gN until
> gN+1 succeeded", which is exactly the foot-gun we are trying avoid with
> 2 closures.
> 
> Separating acquire and use logics not just simpler API to implement (and
> provide), but also more effective compare to your example here. With
> single closure we basically move API responsibility to the users (e.g.,
> do not run this part of the code in the loop, do not access to any data
> behind any guard if all the locks aren't taken yet, etc.), which is not
> a good thing to do, especially from the high-level API.

!!!!!
OK - now I finally understand what I was missing, it totally slipped my mind
that we would have this requirement. One thing I'm not sure this takes into
account though: what about a situation where you can't actually know you need
to acquire gN+1 until you've acquired gN and looked at it? This is at least a
fairly common pattern with KMS, I'm not sure if it comes up with other parts
of the kernel using ww mutexes.

-- 
Cheers,
 Lyude Paul (she/her)
 Senior Software Engineer at Red Hat

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