From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D4B7532E730; Wed, 7 Jan 2026 10:11:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767780716; cv=none; b=S/UQJR4zS0Ta2obtlyyz1k2USRHM+lWmyrz1Ri/jc+iQ8t5hHRM//gXUVoYk+HLWZmb4uC6qp8OYRbgBuwTG+ev0NoDOF1ZsJMSf5BNS9/0457hD/YO14BktnYTrt+PVYgZ1G1OrNS8yLsyX19Jzu3wAnvAhr4az/pzxvFsQYNI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767780716; c=relaxed/simple; bh=089y2IDoSCjwqehlq3MO4nDE6Ee9AOzoaarffxPZP/s=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=bNZtTtCVRkdcSJ7pdpSeS9EgcbBGs/Yj5UICKE520qFL/HvP3R8XVJeitJnk/PKOseztE5jcxMQo35dUwPvRxVBVuUYlzzF8IIj+XmoiLQmAM33hlzFUMvIdKYM5mcKF6T5iRIFgY/0/S72GXUzlSPG0E0UpPsZrVCpX/7Qv1P4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YBursUu1; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YBursUu1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9F0F1C4CEF7; Wed, 7 Jan 2026 10:11:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767780715; bh=089y2IDoSCjwqehlq3MO4nDE6Ee9AOzoaarffxPZP/s=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=YBursUu1NUOpLsViKALRQCFu+cSwuV4OxKTINf6fkb6mUbGc6nvmhTWHuh6spFkzE INghZ7oIQgn06bdINirmASKob38X3/FQsHUos8z/Xn7bcthvscIJcqUbPb3+FjL+/U 3E0fqmhrcfTO4rKTfv3QtJ1pWum/y/w2qVsra9MuD5zoB4rtYP8x1nFK6NOwu5If1Z 653EBTSVQm8E2YTO/WNT33ONMhyv8e2vPKCOryv86s6+Ui7EaThW7NvnjO40Mq30Mg RDQLbFljrB5dGEyE5k0viEwo1r1OMiZ91h+5c1rSEtbeaKWG7VRciDGTveaSCpCJ+T IGfPZNStfJXeA== From: Andreas Hindborg To: FUJITA Tomonori Cc: fujita.tomonori@gmail.com, aliceryhl@google.com, lyude@redhat.com, boqun.feng@gmail.com, will@kernel.org, peterz@infradead.org, richard.henderson@linaro.org, mattst88@gmail.com, linmag7@gmail.com, catalin.marinas@arm.com, ojeda@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, tmgross@umich.edu, dakr@kernel.org, mark.rutland@arm.com, frederic@kernel.org, tglx@linutronix.de, anna-maria@linutronix.de, jstultz@google.com, sboyd@kernel.org, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile In-Reply-To: <20260106.222826.2155269977755242640.fujita.tomonori@gmail.com> References: <20260101.130012.2122315449079707392.fujita.tomonori@gmail.com> <87ikdej4s1.fsf@t14s.mail-host-address-is-not-set> <20260106.222826.2155269977755242640.fujita.tomonori@gmail.com> Date: Wed, 07 Jan 2026 11:11:43 +0100 Message-ID: <87cy3livfk.fsf@t14s.mail-host-address-is-not-set> Precedence: bulk X-Mailing-List: linux-alpha@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain FUJITA Tomonori writes: > On Tue, 06 Jan 2026 13:37:34 +0100 > Andreas Hindborg wrote: > >> "FUJITA Tomonori" writes: >> >>> On Thu, 01 Jan 2026 11:11:23 +0900 (JST) >>> FUJITA Tomonori wrote: >>> >>>> On Wed, 31 Dec 2025 12:22:28 +0000 >>>> Alice Ryhl wrote: >>>> >>>>> Using `READ_ONCE` is the correct way to read the `node.expires` field. >>>>> >>>>> Signed-off-by: Alice Ryhl >>>>> --- >>>>> rust/kernel/time/hrtimer.rs | 8 +++----- >>>>> 1 file changed, 3 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs >>>>> index 856d2d929a00892dc8eaec63cebdf547817953d3..e2b7a26f8aade972356c3eb5f6489bcda3e2e849 100644 >>>>> --- a/rust/kernel/time/hrtimer.rs >>>>> +++ b/rust/kernel/time/hrtimer.rs >>>>> @@ -239,11 +239,9 @@ pub fn expires(&self) -> HrTimerInstant >>>>> // - Timers cannot have negative ktime_t values as their expiration time. >>>>> // - There's no actual locking here, a racy read is fine and expected >>>>> unsafe { >>>>> - Instant::from_ktime( >>>>> - // This `read_volatile` is intended to correspond to a READ_ONCE call. >>>>> - // FIXME(read_once): Replace with `read_once` when available on the Rust side. >>>>> - core::ptr::read_volatile(&raw const ((*c_timer_ptr).node.expires)), >>>>> - ) >>>>> + Instant::from_ktime(kernel::sync::READ_ONCE( >>>>> + &raw const (*c_timer_ptr).node.expires, >>>>> + )) >>>>> } >>>> >>>> Do we actually need READ_ONCE() here? I'm not sure but would it be >>>> better to call the C-side API? >>>> >>>> diff --git a/rust/helpers/time.c b/rust/helpers/time.c >>>> index 67a36ccc3ec4..73162dea2a29 100644 >>>> --- a/rust/helpers/time.c >>>> +++ b/rust/helpers/time.c >>>> @@ -2,6 +2,7 @@ >>>> >>>> #include >>>> #include >>>> +#include >>>> #include >>>> >>>> void rust_helper_fsleep(unsigned long usecs) >>>> @@ -38,3 +39,8 @@ void rust_helper_udelay(unsigned long usec) >>>> { >>>> udelay(usec); >>>> } >>>> + >>>> +__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer) >>>> +{ >>>> + return timer->node.expires; >>>> +} >>> >>> Sorry, of course this should be: >>> >>> +__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer) >>> +{ >>> + return hrtimer_get_expires(timer); >>> +} >>> >> >> This is a potentially racy read. As far as I recall, we determined that >> using read_once is the proper way to handle the situation. >> >> I do not think it makes a difference that the read is done by C code. > > What does "racy read" mean here? > > The C side doesn't use WRITE_ONCE() or READ_ONCE for node.expires. How > would using READ_ONCE() on the Rust side make a difference? Data races like this are UB in Rust. As far as I understand, using this READ_ONCE implementation or a relaxed atomic read would make the read well defined. I am not aware if this is only the case if all writes to the location from C also use atomic operations or WRITE_ONCE. @Boqun? Best regards, Andreas Hindborg