Message-ID: <c19edf2d-2b53-403f-abcc-a5e81e7613f8@gmail.com>
Date: Sun, 13 Oct 2024 19:39:29 +0200
From: Dirk Behme <dirk.behme@...il.com>
To: Boqun Feng <boqun.feng@...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 13.10.24 00:26, Boqun Feng wrote:
> On Sat, Oct 12, 2024 at 09:50:00AM +0200, Dirk Behme wrote:
>> On 12.10.24 09:41, Boqun Feng wrote:
>>> 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).
>>
>>
>> I thought we are talking about anything like
>>
>> #[pin_data]
>> struct ArcIntrusiveTimer {
>>        #[pin]
>>        timer: Timer<Self>,
>>        #[pin]
>> -      flag: SpinLockIrq<u64>,
>> +      flag: u64,
>>        #[pin]
>>        cond: CondVar,
>> }
>>
>> ?
>>
> 
> Yes, but have you tried to actually use that for the example from
> Andreas? I think you will find that you cannot write to `flag` inside
> the timer callback, because you only has a `Arc<ArcIntrusiveTimer>`, so
> not mutable reference for `ArcIntrusiveTimer`. You can of course use
> unsafe to create a mutable reference to `flag`, but it won't be sound,
> since you are getting a mutable reference from an immutable reference.

Yes, of course. But, hmm, wouldn't that unsoundness be independent on 
the topic we discuss here? I mean we are talking about getting the 
compiler to read/modify/write 'flag' in the TimerCallback. *How* we 
tell him to do so should be independent on the result what we want to 
look at regarding the locking requirements of 'flag'?

Anyhow, my root motivation was to simplify Andreas example to not use 
a lock where not strictly required. And with this make Andreas example 
independent on mutex lockdep issues, SpinLockIrq changes and possible 
required CondVar updates. But maybe we find an other way to simplify 
it and decrease the dependencies. In the end its just example code ;)

Best regards

Dirk


> Regards,
> Boqun
> 
>> Best regards
>>
>> Dirk
>>
>>>> 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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