Message-ID: <a5889093869829bebf5626cc508ec575907ffc4f.camel@redhat.com>
Date: Wed, 06 Aug 2025 15:57:41 -0400
From: Lyude Paul <lyude@...hat.com>
To: Benno Lossin <lossin@...nel.org>, rust-for-linux@...r.kernel.org, Thomas
 Gleixner <tglx@...utronix.de>, Boqun Feng <boqun.feng@...il.com>,
 linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Miguel
 Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo
 <gary@...yguo.net>, Björn Roy Baron	
 <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice
 Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich	 <dakr@...nel.org>
Subject: Re: [PATCH v3] rust: lock: Export Guard::do_unlocked()

On Fri, 2025-07-25 at 12:51 +0200, Benno Lossin wrote:
> On Thu Jul 24, 2025 at 11:15 PM CEST, Lyude Paul wrote:
> > In RVKMS, I discovered a silly issue where as a result of our HrTimer for
> > vblank emulation and our vblank enable/disable callbacks sharing a
> > spinlock, it was possible to deadlock while trying to disable the vblank
> > timer.
> > 
> > The solution for this ended up being simple: keep track of when the HrTimer
> > could potentially acquire the shared spinlock, and simply drop the spinlock
> > temporarily from our vblank enable/disable callbacks when stopping the
> > timer. And do_unlocked() ended up being perfect for this.
> 
> In this case, couldn't you also just add another function to stopping
> the timer that takes the lock guard (instead of locking the lock itself)?

This is a nice idea! Unfortunately though stopping the timer is only one of
the spots that we can deadlock. There's another spot that we pretty much only
call once that needs the same behavior and isn't stopping the timer:

   impl HrTimerCallback for VblankTimer {
       type Pointer<'a> = Arc<Self>;
   
       fn run(
           this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>,
           mut context: HrTimerCallbackContext<'_, Self>,
       ) -> HrTimerRestart
       where
           Self: Sized + HasHrTimer<Self>,
       {
           let mut vbl_state = this.crtc.vblank_state.lock();
   
           // Check if we're being asked to stop before continuing
           if vbl_state.cancelling {
               vbl_state.cancelling = false;
               return HrTimerRestart::NoRestart;
           }
   
           let armed = vbl_state.armed.as_ref().expect("VBL state should be armed by now");
   
           if let Some(ref timer) = vbl_state.timer {
               let frame_duration = Delta::from_nanos(armed.frame_duration.into());
   
               let overrun = context.forward_now(frame_duration);
               if overrun != 1 {
                   dev_warn!(
                       this.crtc.drm_dev().as_ref(),
                       "vblank timer overrun (expected 1, got {overrun})\n"
                   );
               }
           }
   
           // Indicate we're about to report the vblank, e.g. enable/disable can't block on this timer
           vbl_state.reporting = true;
   
           // Handle the vblank, dropping the vbl_state lock to prevent a circular locking dependency
           vbl_state.do_unlocked(|| this.crtc.handle_vblank());
   
           vbl_state.reporting = false;
   
           if vbl_state.cancelling {
               vbl_state.cancelling = false;
               HrTimerRestart::NoRestart
           } else {
               HrTimerRestart::Restart
           }
       }
   }

We need to drop the lock here as well because handle_vblank() calls down to
the driver's get_vblank_timestamp() implementation, which in this case also
acquires the lock on this.crtc.vblank_state.

Keep in mind, `handle_vblank()` is a binding for `drm_crtc_handle_vblank()`,
so we can't modify it to take a lock guard or something like that. I guess if
we really needed to we could add our own interface around handle_vblank() for
rvkms, but that seems a bit less straight-forward to me.

> 
> > Since this seems like it's useful, let's export this for use by the rest of
> > the world and write short documentation for it.
> > 
> > Signed-off-by: Lyude Paul <lyude@...hat.com>
> > 
> > ---
> > V2:
> > * Fix documentation for do_unlocked
> > * Add an example
> > V3:
> > * Documentation changes from Miguel
> 
> Please wait a couple days before sending a new version, thanks!
> 
> >  rust/kernel/sync/lock.rs | 35 ++++++++++++++++++++++++++++++++++-
> >  1 file changed, 34 insertions(+), 1 deletion(-)
> > 
> > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > index e82fa5be289c1..800cdd16dce6e 100644
> > --- a/rust/kernel/sync/lock.rs
> > +++ b/rust/kernel/sync/lock.rs
> > @@ -228,7 +228,40 @@ pub fn lock_ref(&self) -> &'a Lock<T, B> {
> >          self.lock
> >      }
> >  
> > -    pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
> > +    /// Releases this [`Guard`]'s lock temporarily, executes `cb` and then re-acquires it.
> > +    ///
> > +    /// This can be useful for situations where you may need to do a temporary unlock dance to avoid
> > +    /// issues like circular locking dependencies.
> > +    ///
> > +    /// It returns the value returned by the closure.
> > +    ///
> > +    /// # Examples
> > +    ///
> > +    /// The following example shows how to use [`Guard::do_unlocked`] to temporarily release a lock,
> > +    /// do some work, then re-lock it.
> 
> I would remove this sentence, as the example is pretty easy to follow
> (at least at the moment).
> 
> > +    ///
> > +    /// ```
> > +    /// # use kernel::{new_spinlock, sync::lock::{Backend, Guard, Lock}};
> > +    /// # use pin_init::stack_pin_init;
> > +    /// fn assert_held<T, B: Backend>(guard: &Guard<'_, T, B>, lock: &Lock<T, B>) {
> > +    ///     // Address-equal means the same lock.
> > +    ///     assert!(core::ptr::eq(guard.lock_ref(), lock));
> > +    /// }
> > +    ///
> > +    /// stack_pin_init! {
> > +    ///     let l = new_spinlock!(42)
> > +    /// }
> 
> I normally write
> 
>     stack_pin_init!(let l = new_spinlock!(42));
> 
> Or do you feel like that's less readable?
> 
> > +    ///
> > +    /// let mut g = l.lock();
> > +    /// let val = *g;
> > +    ///
> > +    /// // The lock will be released, but only temporarily
> > +    /// g.do_unlocked(|| assert_eq!(val, 42));
> 
> I feel like this example doesn't show how `do_unlocked` is useful. How
> about you add a function that locks the lock & you call it from inside
> the `do_unlocked` call? Similar to the issue you described in the commit
> message?
> 
> ---
> Cheers,
> Benno
> 
> > +    ///
> > +    /// // `g` originates from `l` and should be relocked now.
> > +    /// assert_held(&g, &l);
> > +    /// ```
> > +    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) };
> >  
> > 
> > base-commit: dff64b072708ffef23c117fa1ee1ea59eb417807
> 

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

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