Message-ID: <Zwmy-2Yc7vGboYvl@boqun-archlinux>
Date: Fri, 11 Oct 2024 16:21:31 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Dirk Behme <dirk.behme@...il.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, Lyude Paul <lyude@...hat.com>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/14] hrtimer Rust API

On Fri, Oct 11, 2024 at 05:43:57PM +0200, Dirk Behme wrote:
> Hi Andreas,
> 
> Am 11.10.24 um 16:52 schrieb Andreas Hindborg:
> > 
> > Dirk, thanks for reporting!
> 
> :)
> 
> > Boqun Feng <boqun.feng@...il.com> 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?
> 

Reading while there is another CPU is writing is data-race, which is UB.

Regards,
Boqun

> 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@redhat.com/
> > 
> > 
> 

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