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Date:   Tue, 11 Apr 2023 21:47:21 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org,
        Wedson Almeida Filho <walmeida@microsoft.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Subject: Re: [PATCH v4 09/13] rust: add basic `Task`
Message-ID: <20230411214721.2d6f8fda.gary@garyguo.net>
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        <20230411054543.21278-9-wedsonaf@gmail.com>
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On Tue, 11 Apr 2023 02:45:39 -0300
Wedson Almeida Filho <wedsonaf@gmail.com> wrote:

> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> It is an abstraction for C's `struct task_struct`. It implements
> `AlwaysRefCounted`, so the refcount of the wrapped object is managed
> safely on the Rust side.
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> ---
> v1 -> v2: No changes
> v2 -> v3: Wrap task_struct with `Opaque` instead of `UnsafeCell`
> v3 -> v4: No changes
> 
>  rust/bindings/bindings_helper.h |  1 +
>  rust/helpers.c                  | 19 +++++++++
>  rust/kernel/lib.rs              |  1 +
>  rust/kernel/task.rs             | 75 +++++++++++++++++++++++++++++++++
>  4 files changed, 96 insertions(+)
>  create mode 100644 rust/kernel/task.rs
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 75d85bd6c592..03656a44a83f 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -8,6 +8,7 @@
>  
>  #include <linux/slab.h>
>  #include <linux/refcount.h>
> +#include <linux/sched.h>
>  
>  /* `bindgen` gets confused at certain things. */
>  const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
> diff --git a/rust/helpers.c b/rust/helpers.c
> index efd5b1e86f6e..f545923aedd8 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -24,6 +24,7 @@
>  #include <linux/refcount.h>
>  #include <linux/mutex.h>
>  #include <linux/spinlock.h>
> +#include <linux/sched/signal.h>
>  
>  __noreturn void rust_helper_BUG(void)
>  {
> @@ -76,6 +77,12 @@ void rust_helper_spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_spin_unlock_irqrestore);
>  
> +int rust_helper_signal_pending(struct task_struct *t)
> +{
> +	return signal_pending(t);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_signal_pending);
> +
>  refcount_t rust_helper_REFCOUNT_INIT(int n)
>  {
>  	return (refcount_t)REFCOUNT_INIT(n);
> @@ -112,6 +119,18 @@ long rust_helper_PTR_ERR(__force const void *ptr)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_PTR_ERR);
>  
> +void rust_helper_get_task_struct(struct task_struct *t)
> +{
> +	get_task_struct(t);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_get_task_struct);
> +
> +void rust_helper_put_task_struct(struct task_struct *t)
> +{
> +	put_task_struct(t);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
> +
>  /*
>   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
>   * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 2d7606135ef6..ee27e10da479 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -44,6 +44,7 @@ mod static_assert;
>  pub mod std_vendor;
>  pub mod str;
>  pub mod sync;
> +pub mod task;
>  pub mod types;
>  
>  #[doc(hidden)]
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> new file mode 100644
> index 000000000000..d70cad131956
> --- /dev/null
> +++ b/rust/kernel/task.rs
> @@ -0,0 +1,75 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Tasks (threads and processes).
> +//!
> +//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
> +
> +use crate::{bindings, types::Opaque};
> +use core::ptr;
> +
> +/// Wraps the kernel's `struct task_struct`.
> +///
> +/// # Invariants
> +///
> +/// All instances are valid tasks created by the C portion of the kernel.
> +///
> +/// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
> +/// that the allocation remains valid at least until the matching call to `put_task_struct`.
> +#[repr(transparent)]
> +pub struct Task(pub(crate) Opaque<bindings::task_struct>);
> +
> +// SAFETY: It's OK to access `Task` through references from other threads because we're either
> +// accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
> +// synchronised by C code (e.g., `signal_pending`).
> +unsafe impl Sync for Task {}
> +
> +/// The type of process identifiers (PIDs).
> +type Pid = bindings::pid_t;

This'll just end up being `i32`. I think it'll be more desirable that
this is defined as a new type.

> +
> +impl Task {
> +    /// Returns the group leader of the given task.
> +    pub fn group_leader(&self) -> &Task {
> +        // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
> +        // have a valid group_leader.
> +        let ptr = unsafe { *ptr::addr_of!((*self.0.get()).group_leader) };
> +
> +        // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
> +        // and given that a task has a reference to its group leader, we know it must be valid for
> +        // the lifetime of the returned task reference.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    /// Returns the PID of the given task.
> +    pub fn pid(&self) -> Pid {
> +        // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
> +        // have a valid pid.
> +        unsafe { *ptr::addr_of!((*self.0.get()).pid) }
> +    }
> +
> +    /// Determines whether the given task has pending signals.
> +    pub fn signal_pending(&self) -> bool {
> +        // SAFETY: By the type invariant, we know that `self.0` is valid.
> +        unsafe { bindings::signal_pending(self.0.get()) != 0 }
> +    }
> +
> +    /// Wakes up the task.
> +    pub fn wake_up(&self) {
> +        // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
> +        // And `wake_up_process` is safe to be called for any valid task, even if the task is
> +        // running.
> +        unsafe { bindings::wake_up_process(self.0.get()) };
> +    }
> +}
> +
> +// SAFETY: The type invariants guarantee that `Task` is always ref-counted.
> +unsafe impl crate::types::AlwaysRefCounted for Task {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> +        unsafe { bindings::get_task_struct(self.0.get()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
> +        unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
> +    }
> +}