Re: [PATCH v4] rust: lock: Export Guard::do_unlocked()

[Lyude Paul <lyude@redhat.com>]

On Thu, 2025-10-30 at 11:43 +0100, Paolo Bonzini wrote:
> On 10/29/25 19:35, Lyude Paul wrote:
> > +    /// // Since we hold work.lock, which work will also try to acquire in WorkItem::run. Dropping
> > +    /// // the lock temporarily while we wait for completion works around this.
> > +    /// g.do_unlocked(|| work.done.wait_for_completion());
> > +    ///
> > +    /// assert_eq!(*g, 42);
> > +    /// ```
> > +    pub fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
> >           // SAFETY: The caller owns the lock, so it is safe to unlock it.
> >           unsafe { B::unlock(self.lock.state.get(), &self.state) };
> 
> Getting self as &mut is incorrect.  That's because owning a lock guard 
> implicitly tells you that no other thread can observe the intermediate 
> states of the object.  (The same is even more obviously true for a 
> RefCell's mutable borrow, i.e. core::cell::RefMut)
> 
> Let's say you have a lock-protected data structure with an invariant 
> that is preserved at the end of every critical section.  Let's say also 
> that you have a function
> 
>      fn do_something() {
>          let g = self.inner.lock();
>          g.mess_up_the_invariant();          // (1)
>          self.do_something_else(&mut g);     // uses do_unlocked()
>          g.fix_the_invariant();              // (2)
>      }
> 
> Because the function holds a guard between the calls (1) and (2), it 
> expects that other thread cannot observe the temporary state.  The fact 
> that do_unlocked() takes a &mut doesn't help, because the common case 
> for RAII objects is that they're passed around mutably.
> 
> Instead, do_unlocked should take the guard and return another one:
> 
>      fn do_something() {
>          let mut g = self.inner.lock();
>          g.mess_up_the_invariant();          // (1)
>          g = self.do_something_else(g);      // uses do_unlocked()
>          g.fix_the_invariant();              // (2)
>      }
> 
> This version of the interface makes it clear that (1) and (2) are in a 
> separate critical section.  Unfortunately it makes the signature uglier 
> for do_unlocked() itself:
> 
>      #[must_use]
>      pub fn do_unlocked<U>(self, cb: impl FnOnce() -> U) -> (Self, U)

Hm, it seems then that we should probably fix this before exporting it then!
Thank you for pointing this out, I'll fix it in the next respin.


> 
> Paolo

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

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