Re: [PATCH 2/3] rust: sync: Assert Lock::is_locked in Guard::new for debug builds

[Lyude Paul <lyude@redhat.com>]

On Wed, 2024-11-20 at 15:59 -0800, Boqun Feng wrote:
> On Wed, Nov 20, 2024 at 05:30:42PM -0500, Lyude Paul wrote:
> > Since we're allowing code to unsafely claim that it's acquired a lock
> > let's use the new Lock::is_locked() function so that when debug assertions
> > are enabled, we can verify that the lock has actually been acquired.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> >  rust/kernel/sync/lock.rs | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > index 542f846ac02b2..0a7f2ed767423 100644
> > --- a/rust/kernel/sync/lock.rs
> > +++ b/rust/kernel/sync/lock.rs
> > @@ -244,10 +244,17 @@ fn drop(&mut self) {
> >  impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
> >      /// Constructs a new immutable lock guard.
> >      ///
> > +    /// # Panics
> > +    ///
> > +    /// This function will panic if debug assertions are enabled and `lock` is not actually
> > +    /// acquired.
> > +    ///
> >      /// # Safety
> >      ///
> >      /// The caller must ensure that it owns the lock.
> >      pub unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
> > +        debug_assert!(lock.is_locked());
> 
> You should just use lockdep_assert_held() here, and there's no need for
> new_unchecked().

I'm fine using lockdep for this, I guess I'm curious - wouldn't we still want
to at least avoid this lockdep check when we explicitly just grabbed the lock?
Or do we just not really care too much about the performance case of being
under lockdep (which is reasonable enough :)

> 
> Regards,
> Boqun
> 
> > +
> >          Self {
> >              lock,
> >              state,
> > -- 
> > 2.47.0
> > 
> 

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

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