Re: [RFC PATCH 3/5] rust: block: allow `hrtimer::Timer` in `RequestData`

From: Benno Lossin <benno.lossin@proton.me>
To: Andreas Hindborg <nmi@metaspace.dk>
Cc: "Jens Axboe" <axboe@kernel.dk>, "Christoph Hellwig" <hch@lst.de>,
	"Keith Busch" <kbusch@kernel.org>,
	"Damien Le Moal" <Damien.LeMoal@wdc.com>,
	"Bart Van Assche" <bvanassche@acm.org>,
	"Hannes Reinecke" <hare@suse.de>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Niklas Cassel" <Niklas.Cassel@wdc.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	"Gary Guo" <gary@garyguo.net>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Chaitanya Kulkarni" <chaitanyak@nvidia.com>,
	"Luis Chamberlain" <mcgrof@kernel.org>,
	"Yexuan Yang" <1182282462@bupt.edu.cn>,
	"Sergio González Collado" <sergio.collado@gmail.com>,
	"Joel Granados" <j.granados@samsung.com>,
	"Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>,
	"Daniel Gomez" <da.gomez@samsung.com>,
	"open list" <linux-kernel@vger.kernel.org>,
	"rust-for-linux@vger.kernel.org" <rust-for-linux@vger.kernel.org>,
	"lsf-pc@lists.linux-foundation.org"
	<lsf-pc@lists.linux-foundation.org>,
	"gost.dev@samsung.com" <gost.dev@samsung.com>
Subject: Re: [RFC PATCH 3/5] rust: block: allow `hrtimer::Timer` in `RequestData`
Date: Sat, 23 Mar 2024 10:51:57 +0000	[thread overview]
Message-ID: <sHRsbsEAgQZOgBjItL1A-a1BOEGTeH4CWqZsrdny4vCI06o56pmKluCTbY_EwRDO1hCjEp9Cuq9_8S8Co2I9c8wquHWZH_KGjOpmbF1YiJc=@proton.me> (raw)
In-Reply-To: <20240313110515.70088-4-nmi@metaspace.dk>

On 3/13/24 12:05, Andreas Hindborg wrote:> From: Andreas Hindborg <a.hindborg@samsung.com>
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
> ---
>   rust/kernel/block/mq/request.rs | 67 ++++++++++++++++++++++++++++++++-
>   1 file changed, 66 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
> index cccffde45981..8b7f08f894be 100644
> --- a/rust/kernel/block/mq/request.rs
> +++ b/rust/kernel/block/mq/request.rs
> @@ -4,13 +4,16 @@
>   //!
>   //! C header: [`include/linux/blk-mq.h`](srctree/include/linux/blk-mq.h)
> 
> +use kernel::hrtimer::RawTimer;
> +
>   use crate::{
>       bindings,
>       block::mq::Operations,
>       error::{Error, Result},
> +    hrtimer::{HasTimer, TimerCallback},
>       types::{ARef, AlwaysRefCounted, Opaque},
>   };
> -use core::{ffi::c_void, marker::PhantomData, ops::Deref};
> +use core::{ffi::c_void, marker::PhantomData, ops::Deref, ptr::NonNull};
> 
>   use crate::block::bio::Bio;
>   use crate::block::bio::BioIterator;
> @@ -175,6 +178,68 @@ fn deref(&self) -> &Self::Target {
>       }
>   }
> 
> +impl<T> RawTimer for RequestDataRef<T>
> +where
> +    T: Operations,
> +    T::RequestData: HasTimer<T::RequestData>,
> +    T::RequestData: Sync,
> +{
> +    fn schedule(self, expires: u64) {
> +        let self_ptr = self.deref() as *const T::RequestData;
> +        core::mem::forget(self);
> +
> +        // SAFETY: `self_ptr` is a valid pointer to a `T::RequestData`
> +        let timer_ptr = unsafe { T::RequestData::raw_get_timer(self_ptr) };
> +
> +        // `Timer` is `repr(transparent)`
> +        let c_timer_ptr = timer_ptr.cast::<bindings::hrtimer>();
> +
> +        // Schedule the timer - if it is already scheduled it is removed and
> +        // inserted
> +
> +        // SAFETY: c_timer_ptr points to a valid hrtimer instance that was
> +        // initialized by `hrtimer_init`
> +        unsafe {
> +            bindings::hrtimer_start_range_ns(
> +                c_timer_ptr as *mut _,
> +                expires as i64,
> +                0,
> +                bindings::hrtimer_mode_HRTIMER_MODE_REL,
> +            );
> +        }
> +    }
> +}
> +
> +impl<T> kernel::hrtimer::RawTimerCallback for RequestDataRef<T>
> +where
> +    T: Operations,
> +    T: Sync,

Why is this needed? Shouldn't this be `T::RequestData: Sync`?

Is the `run` function below executed on a different thread compared to
the `schedule` function above?
If yes, then `T::RequestData` probably also needs to be `Send`.
You also would need to adjust the bounds in the impl above.

-- 
Cheers,
Benno

> +    T::RequestData: HasTimer<T::RequestData>,
> +    T::RequestData: TimerCallback<Receiver = Self>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
> +        // `Timer` is `repr(transparent)`
> +        let timer_ptr = ptr.cast::<kernel::hrtimer::Timer<T::RequestData>>();
> +
> +        // SAFETY: By C API contract `ptr` is the pointer we passed when
> +        // enqueing the timer, so it is a `Timer<T::RequestData>` embedded in a `T::RequestData`
> +        let receiver_ptr = unsafe { T::RequestData::timer_container_of(timer_ptr) };
> +
> +        // SAFETY: The pointer was returned by `T::timer_container_of` so it
> +        // points to a valid `T::RequestData`
> +        let request_ptr = unsafe { bindings::blk_mq_rq_from_pdu(receiver_ptr.cast::<c_void>()) };
> +
> +        // SAFETY: We own a refcount that we leaked during `RawTimer::schedule()`
> +        let dref = RequestDataRef::new(unsafe {
> +            ARef::from_raw(NonNull::new_unchecked(request_ptr.cast::<Request<T>>()))
> +        });
> +
> +        T::RequestData::run(dref);
> +
> +        bindings::hrtimer_restart_HRTIMER_NORESTART
> +    }
> +}
> +
>   // SAFETY: All instances of `Request<T>` are reference counted. This
>   // implementation of `AlwaysRefCounted` ensure that increments to the ref count
>   // keeps the object alive in memory at least until a matching reference count
> --
> 2.44.0
>