linux-kernel - Re: [PATCH v4 4/7] rust: time: Add wrapper for fsleep function

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Message-Id: <20241029.083029.72679397436968362.fujita.tomonori@gmail.com>
Date: Tue, 29 Oct 2024 08:30:29 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: boqun.feng@...il.com
Cc: fujita.tomonori@...il.com, anna-maria@...utronix.de,
 frederic@...nel.org, tglx@...utronix.de, jstultz@...gle.com,
 sboyd@...nel.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 rust-for-linux@...r.kernel.org, andrew@...n.ch, hkallweit1@...il.com,
 tmgross@...ch.edu, ojeda@...nel.org, alex.gaynor@...il.com,
 gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
 a.hindborg@...sung.com, aliceryhl@...gle.com, arnd@...db.de
Subject: Re: [PATCH v4 4/7] rust: time: Add wrapper for fsleep function

On Sun, 27 Oct 2024 21:38:41 -0700
Boqun Feng <boqun.feng@...il.com> wrote:

> On Mon, Oct 28, 2024 at 09:50:30AM +0900, FUJITA Tomonori wrote:
>> On Fri, 25 Oct 2024 15:03:37 -0700
>> Boqun Feng <boqun.feng@...il.com> wrote:
>> 
>> >> +/// Sleeps for a given duration at least.
>> >> +///
>> >> +/// Equivalent to the kernel's [`fsleep`], flexible sleep function,
>> >> +/// which automatically chooses the best sleep method based on a duration.
>> >> +///
>> >> +/// The function sleeps infinitely (MAX_JIFFY_OFFSET) if `Delta` is negative
>> >> +/// or exceedes i32::MAX milliseconds.
>> >> +///
>> > 
>> > I know Miguel has made his suggestion:
>> > 
>> > 	https://lore.kernel.org/rust-for-linux/CANiq72kWqSCSkUk1efZyAi+0ScNTtfALn+wiJY_aoQefu2TNvg@mail.gmail.com/
>> > 
>> > , but I think what we should really do here is just panic if `Delta` is
>> > negative or exceedes i32::MAX milliseconds, and document clearly that
>> > this function expects `Delta` to be in a certain range, i.e. it's the
>> > user's responsibility to check. Because:
>> > 
>> > *	You can simply call schedule() with task state set properly to
>> > 	"sleep infinitely".
>> > 
>> > *	Most of the users of fsleep() don't need this "sleep infinitely"
>> > 	functionality. Instead, they want to sleep with a reasonable
>> > 	short time.
>> 
>> I agree with the above reasons but I'm not sure about just panic with
>> a driver's invalid argument.
>> 
> 
> If a driver blindly trusts a user-space input or a value chosen by the
> hardware, I would say it's a bug in the driver. So IMO drivers should
> check the input of fsleep().
> 
>> Can we just return an error instead?
>> 
> 
> That also works for me, but an immediate question is: do we put
> #[must_use] on `fsleep()` to enforce the use of the return value? If
> yes, then the normal users would need to explicitly ignore the return
> value:
> 
> 	let _ = fsleep(1sec);
> 
> The "let _ =" would be a bit annoying for every user that just uses a
> constant duration.

Yeah, but I don't think that we have enough of an excuse here to break
the rule "Do not crash the kernel".

Another possible option is to convert an invalid argument to a safe
value (e.g., the maximum), possibly with WARN_ON_ONCE().