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[2003:df:bf0d:b400:7350:36ae:6e0c:c598]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a99f3ed51e8sm15282566b.30.2024.10.12.00.50.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 12 Oct 2024 00:50:01 -0700 (PDT) Message-ID: <4dd93603-04fa-4da4-b867-bd12ece4b391@gmail.com> Date: Sat, 12 Oct 2024 09:50:00 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 00/14] hrtimer Rust API To: Boqun Feng Cc: Andreas Hindborg , Lyude Paul , Dirk Behme , Miguel Ojeda , Alex Gaynor , Anna-Maria Behnsen , Frederic Weisbecker , Thomas Gleixner , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Alice Ryhl , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org References: <20240917222739.1298275-1-a.hindborg@kernel.org> <87a5falmjy.fsf@kernel.org> Content-Language: de-AT-frami From: Dirk Behme In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 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, >>>>>> +//! #[pin] >>>>>> +//! flag: Mutex, >>>>>> +//! #[pin] >>>>>> +//! cond: CondVar, >>>>>> +//! } >>>>>> >>>>>> has a Mutex, 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, #[pin] - flag: SpinLockIrq, + flag: u64, #[pin] cond: CondVar, } ? 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, >>>>> #[pin] >>>>> flag: SpinLockIrq, >>>>> #[pin] >>>>> cond: CondVar, >>>>> } >>>>> >>>>> impl ArcIntrusiveTimer { >>>>> fn new() -> impl PinInit { >>>>> try_pin_init!(Self { >>>>> timer <- Timer::new(), >>>>> flag <- new_spinlock_irq!(0), >>>>> cond <- new_condvar!(), >>>>> }) >>>>> } >>>>> } >>>>> >>>>> impl TimerCallback for ArcIntrusiveTimer { >>>>> type CallbackTarget<'a> = Arc; >>>>> 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 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/ >>>>> >>>>> >>>> >>