Re: [PATCH v2 00/14] hrtimer Rust API

[Dirk Behme]

Hi Andreas,

Am 11.10.24 um 16:52 schrieb Andreas Hindborg:
> Dirk, thanks for reporting!

:)

> Boqun Feng <boqun.feng@xxxxxxxxx> writes:
>
> > On Tue, Oct 01, 2024 at 02:37:46PM +0200, Dirk Behme wrote:
> > > On 18.09.2024 00:27, Andreas Hindborg wrote:
> > > > Hi!
> > > >
> > > > This series adds support for using the `hrtimer` subsystem from Rust code.
> > > >
> > > > I tried breaking up the code in some smaller patches, hopefully that will
> > > > ease the review process a bit.
> > >
> > > Just fyi, having all 14 patches applied I get [1] on the first (doctest)
> > > Example from hrtimer.rs.
> > >
> > > This is from lockdep:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/locking/lockdep.c#n4785
> > >
> > > Having just a quick look I'm not sure what the root cause is. Maybe mutex in
> > > interrupt context? Or a more subtle one?
> >
> > I think it's calling mutex inside an interrupt context as shown by the
> > callstack:
> >
> > ]  __mutex_lock+0xa0/0xa4
> > ] ...
> > ]  hrtimer_interrupt+0x1d4/0x2ac
> >
> > , it is because:
> >
> > +//! struct ArcIntrusiveTimer {
> > +//!     #[pin]
> > +//!     timer: Timer<Self>,
> > +//!     #[pin]
> > +//!     flag: Mutex<bool>,
> > +//!     #[pin]
> > +//!     cond: CondVar,
> > +//! }
> >
> > has a Mutex<bool>, which actually should be a SpinLockIrq [1]. Note that
> > irq-off is needed for the lock, because otherwise we will hit a self
> > deadlock due to interrupts:
> >
> > 	spin_lock(&a);
> > 	> timer interrupt
> > 	  spin_lock(&a);
> >
> > Also notice that the IrqDisabled<'_> token can be simply created by
> > ::new(), because irq contexts should guarantee interrupt disabled (i.e.
> > we don't support nested interrupts*).
>
> I updated the example based on the work in [1]. I think we need to
> update `CondVar::wait` to support waiting with irq disabled. 

Yes, I agree. This answers one of the open questions I had in the 
discussion with Boqun :)

What do you think regarding the other open question: In this *special* 
case here, what do you think to go *without* any lock? I mean the 
'while *guard != 5' loop in the main thread is read only regarding 
guard. So it doesn't matter if it *reads* the old or the new value. 
And the read/modify/write of guard in the callback is done with 
interrupts disabled anyhow as it runs in interrupt context. And with 
this can't be interrupted (excluding nested interrupts). So this 
modification of guard doesn't need to be protected from being 
interrupted by a lock if there is no modifcation of guard "outside" 
the interupt locked context.

What do you think?

Thanks

Dirk


> Without
> this, when we get back from `bindings::schedule_timeout` in
> `CondVar::wait_internal`, interrupts are enabled:
>
> ```rust
> use kernel::{
>      hrtimer::{Timer, TimerCallback, TimerPointer, TimerRestart},
>      impl_has_timer, new_condvar, new_spinlock, new_spinlock_irq,
>      irq::IrqDisabled,
>      prelude::*,
>      sync::{Arc, ArcBorrow, CondVar, SpinLock, SpinLockIrq},
>      time::Ktime,
> };
>
> #[pin_data]
> struct ArcIntrusiveTimer {
>      #[pin]
>      timer: Timer<Self>,
>      #[pin]
>      flag: SpinLockIrq<u64>,
>      #[pin]
>      cond: CondVar,
> }
>
> impl ArcIntrusiveTimer {
>      fn new() -> impl PinInit<Self, kernel::error::Error> {
>          try_pin_init!(Self {
>              timer <- Timer::new(),
>              flag <- new_spinlock_irq!(0),
>              cond <- new_condvar!(),
>          })
>      }
> }
>
> impl TimerCallback for ArcIntrusiveTimer {
>      type CallbackTarget<'a> = Arc<Self>;
>      type CallbackTargetParameter<'a> = ArcBorrow<'a, Self>;
>
>      fn run(this: Self::CallbackTargetParameter<'_>, irq: IrqDisabled<'_>) -> TimerRestart {
>          pr_info!("Timer called\n");
>          let mut guard = this.flag.lock_with(irq);
>          *guard += 1;
>          this.cond.notify_all();
>          if *guard == 5 {
>              TimerRestart::NoRestart
>          }
>          else {
>              TimerRestart::Restart
>
>          }
>      }
> }
>
> impl_has_timer! {
>      impl HasTimer<Self> for ArcIntrusiveTimer { self.timer }
> }
>
>
> let has_timer = Arc::pin_init(ArcIntrusiveTimer::new(), GFP_KERNEL)?;
> let _handle = has_timer.clone().schedule(Ktime::from_ns(200_000_000));
>
> kernel::irq::with_irqs_disabled(|irq| {
>    let mut guard = has_timer.flag.lock_with(irq);
>
>    while *guard != 5 {
>        pr_info!("Not 5 yet, waiting\n");
>        has_timer.cond.wait(&mut guard); // <-- we arrive back here with interrupts enabled!
>    }
> });
> ```
>
> I think an update of `CondVar::wait` should be part of the patch set [1].
>
>
> Best regards,
> Andreas
>
>
> [1] https://lore.kernel.org/rust-for-linux/20240916213025.477225-1-lyude@xxxxxxxxxx/