linux-kernel - Re: [PATCH] rust: sync: add accessor for the lock behind a given guard

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Message-ID: <CAH5fLggDna0E9MtarHDxoS=w3zp-2HipASOLGfYFzTg8fxUu2A@mail.gmail.com>
Date: Tue, 4 Feb 2025 14:39:33 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: sync: add accessor for the lock behind a given guard

On Thu, Jan 30, 2025 at 6:15 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Thu, Jan 30, 2025 at 04:43:22PM +0100, Alice Ryhl wrote:
> > On Thu, Jan 30, 2025 at 4:33 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > >
> > > Hi Alice,
> > >
> > > The patch looks reasonable to me, however...
> > >
> > > On Thu, Jan 30, 2025 at 11:39:32AM +0000, Alice Ryhl wrote:
> > > > Binder has some methods where the caller provides a lock guard, and
> > > > Binder needs to be able to assert that the guard is associated with the
> > > > right lock. To enable this, add an accessor to obtain a reference to the
> > > > underlying lock that you can pass to `ptr::eq`.
> > > >
> > >
> > > ... could you provide more details on this usage, for example, why do
> > > you need the assertion, is it for debug purposes? Does the current C
> > > code have the same or similar logic? Besides...
> >
> > I added the assertion because it makes the SAFETY comment on a call to
> > kernel::list::List::remove simpler. The C driver does not have the
> > assertion.
> >
>
> Ok. Make sense.
>
> > > > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> > > > ---
> > > >  rust/kernel/sync/lock.rs | 7 ++++++-
> > > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > > > index 41dcddac69e2..681d67275b49 100644
> > > > --- a/rust/kernel/sync/lock.rs
> > > > +++ b/rust/kernel/sync/lock.rs
> > > > @@ -169,7 +169,12 @@ pub struct Guard<'a, T: ?Sized, B: Backend> {
> > > >  // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
> > > >  unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
> > > >
> > > > -impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
> > > > +impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
> > > > +    /// Returns the lock that this guard originates from.
> > > > +    pub fn lock(&self) -> &'a Lock<T, B> {
> > >
> > > How about we name this as `lock_ref()` or something else, because
> > > `lock()` itself is already used by `Lock` for the lock *operation*, and
> > > this is just an accessor, I would like we don't confuse code readers
> > > when they see code like "let b = a.lock()".
> >
> > The usual name for this operation in userspace is "mutex".
> > https://docs.rs/lock_api/0.4.12/lock_api/struct.MutexGuard.html#method.mutex
> >
> > But since our code is generic over many lock types, I went for "lock".
> > But I guess it could make sense to rename it.
> >
>
> Got it. The good thing about the naming of lock_api is that the name
> "mutex" is not used for other purpose, while "lock" is a bit different.
>
> > > Moreover, if the only usage
> > > of this is for asserting the right lock, maybe we can instead add an:
> > >
> > >     pub fn assert_lock_associated(&self, lock_ptr: *const Lock<T, B>)
> >
> > I guess, though there is precedent for having a method that gets the
> > underlying lock, so I think it makes sense. If we had an assertion, it
>
> I don't disagree, but I just feel we should be careful about introducing
> two "lock()" that one is an operation and the other is an accessor.
>
> > would probably take an &Lock<T,B>.
> >
>
> How about:
>
>     impl<T, B: Backend> Lock<T, B> {
>         pub fn assert_held_then<O>(
>             &self, proof: &Guard<'_, T, B>, f: FnOnce() -> O
>         ) -> O {
>             <assert `proof` is associated with `&self`>
>             f()
>         }
>     }
>
> In this way, not only we can assert the correct lock is held, but we can
> also guarantee `f()` is called with the lock held. Thoughts?

I need mutable access to the guard during the function, though? I
don't think a closure is helpful in this case.

How about I rename to `lock_ref()` instead?

Alice