Message-ID: <ZwooHrqIiirl1so7@boqun-archlinux>
Date: Sat, 12 Oct 2024 00:41:18 -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 Sat, Oct 12, 2024 at 07:19:41AM +0200, Dirk Behme wrote:
> On 12.10.24 01:21, Boqun Feng wrote:
> > 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.
> 
> Could you help to understand where exactly you see UB in Andreas' 'while
> *guard != 5' loop in case no locking is used? As mentioned I'm under the

Sure, but could you provide the code of what you mean exactly, if you
don't use a lock here, you cannot have a guard. I need to the exact code
to point out where the compiler may "mis-compile" (a result of being
UB).

> impression that it doesn't matter if the old or new guard value is read in
> this special case.
> 

For one thing, if the compiler believes no one is accessing the value
because the code uses an immutable reference, it can "optimize" the loop
away:

	while *var != 5 {
	    do_something();
	}

into
	
	if *var != 5 {
	    loop { do_something(); }
	}

But as I said, I need to see the exact code to suggest a relevant
mis-compile, and note that sometimes, even mis-compile seems impossible
at the moment, a UB is a UB, compilers are free to do anything they
want (or don't want). So "mis-compile" is only helping we understand the
potential result of a UB.

Regards,
Boqun

> Best regards
> 
> Dirk
> 
> 
> > 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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