linux-kernel - Re: [PATCH net-next v3 5/8] rust: time: Add wrapper for fsleep function

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Message-Id: <20241023.213345.2086786446806395897.fujita.tomonori@gmail.com>
Date: Wed, 23 Oct 2024 21:33:45 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: aliceryhl@...gle.com
Cc: fujita.tomonori@...il.com, 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, anna-maria@...utronix.de, frederic@...nel.org,
 tglx@...utronix.de, arnd@...db.de, jstultz@...gle.com, sboyd@...nel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 5/8] rust: time: Add wrapper for fsleep
 function

On Wed, 16 Oct 2024 10:29:12 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

>> diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
>> new file mode 100644
>> index 000000000000..dc7e2b3a0ab2
>> --- /dev/null
>> +++ b/rust/kernel/time/delay.rs
>> @@ -0,0 +1,31 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! Delay and sleep primitives.
>> +//!
>> +//! This module contains the kernel APIs related to delay and sleep that
>> +//! have been ported or wrapped for usage by Rust code in the kernel.
>> +//!
>> +//! C header: [`include/linux/delay.h`](srctree/include/linux/delay.h).
>> +
>> +use crate::time;
>> +use core::ffi::c_ulong;
>> +
>> +/// 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.
>> +///
>> +/// `Delta` must be longer than one microsecond.
> 
> Why is this required? Right now you just round up to one microsecond,
> which seems okay.

Oops, it should have been removed.

>> +/// This function can only be used in a nonatomic context.
>> +pub fn fsleep(delta: time::Delta) {
>> +    // SAFETY: FFI call.
>> +    unsafe {
>> +        // Convert the duration to microseconds and round up to preserve
>> +        // the guarantee; fsleep sleeps for at least the provided duration,
>> +        // but that it may sleep for longer under some circumstances.
>> +        bindings::fsleep(
>> +            ((delta.as_nanos() + time::NSEC_PER_USEC - 1) / time::NSEC_PER_USEC) as c_ulong,
> 
> You probably want this:
> 
> delta.as_nanos().saturating_add(time::NSEC_PER_USEC - 1) / time::NSEC_PER_USEC
> 
> This would avoid a crash if someone passes i64::MAX nanoseconds and
> CONFIG_RUST_OVERFLOW_CHECKS is enabled.

Ah, I'll fix.

Thanks!