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Wed, 05 Nov 2025 15:02:31 -0800 (PST) From: Mitchell Levy Date: Wed, 05 Nov 2025 15:01:17 -0800 Subject: [PATCH v4 5/9] rust: percpu: introduce a rust API for dynamic per-CPU variables Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20251105-rust-percpu-v4-5-984b1470adcb@gmail.com> References: <20251105-rust-percpu-v4-0-984b1470adcb@gmail.com> In-Reply-To: <20251105-rust-percpu-v4-0-984b1470adcb@gmail.com> To: Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Andrew Morton , Dennis Zhou , Tejun Heo , Christoph Lameter , Danilo Krummrich , Benno Lossin , Yury Norov , Viresh Kumar Cc: Tyler Hicks , Allen Pais , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-mm@kvack.org, Mitchell Levy X-Mailer: b4 0.14.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1762383744; l=9789; i=levymitchell0@gmail.com; s=20240719; h=from:subject:message-id; bh=mFg9QLMAwyqIxa/N6WNggn20WoF/wqWYILNGfUHwvYw=; b=rC6LvrXQz84xbSb5BvGzEQo3PK2NO2XHoHUt+ISh2JaHvuW2RktvXy7YCgzUy8bvkJ1wqFQWO bxVWrfmsXOSBxGpV0RbAHoOa2jK6XivWxBpY5qWUYmJnPND48kmTwbK X-Developer-Key: i=levymitchell0@gmail.com; a=ed25519; pk=n6kBmUnb+UNmjVkTnDwrLwTJAEKUfs2e8E+MFPZI93E= Dynamically allocated per-CPU variables are core to many of the use-cases of per-CPU variables (e.g., ref counting). Add support for them using the core `PerCpuPtr` primitive, implementing the `PerCpu` trait. Co-developed-by: Boqun Feng Signed-off-by: Boqun Feng Signed-off-by: Mitchell Levy --- rust/helpers/percpu.c | 10 ++++ rust/kernel/percpu.rs | 30 ++++++++-- rust/kernel/percpu/dynamic.rs | 130 ++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 166 insertions(+), 4 deletions(-) diff --git a/rust/helpers/percpu.c b/rust/helpers/percpu.c index a091389f730f..35656333dfae 100644 --- a/rust/helpers/percpu.c +++ b/rust/helpers/percpu.c @@ -7,3 +7,13 @@ void __percpu *rust_helper_alloc_percpu(size_t sz, size_t align) return __alloc_percpu(sz, align); } +void *rust_helper_per_cpu_ptr(void __percpu *ptr, unsigned int cpu) +{ + return per_cpu_ptr(ptr, cpu); +} + +void rust_helper_on_each_cpu(smp_call_func_t func, void *info, int wait) +{ + on_each_cpu(func, info, wait); +} + diff --git a/rust/kernel/percpu.rs b/rust/kernel/percpu.rs index 2fba9a165636..294b8ffc4f62 100644 --- a/rust/kernel/percpu.rs +++ b/rust/kernel/percpu.rs @@ -1,14 +1,19 @@ // SPDX-License-Identifier: GPL-2.0 //! Per-CPU variables. //! -//! See the [`crate::define_per_cpu!`] macro and the [`PerCpu`] trait. +//! See the [`crate::define_per_cpu!`] macro, the [`DynamicPerCpu`] type, and the [`PerCpu`] +//! trait. pub mod cpu_guard; +mod dynamic; mod static_; +#[doc(inline)] +pub use dynamic::*; #[doc(inline)] pub use static_::*; +use crate::cpu::CpuId; use crate::declare_extern_per_cpu; use crate::percpu::cpu_guard::CpuGuard; use crate::types::Opaque; @@ -123,6 +128,23 @@ pub fn get_ptr(&self) -> *mut MaybeUninit { // the invariant that self.0 is a valid offset into the per-CPU area. (this_cpu_area).wrapping_add(self.0 as usize).cast() } + + /// Get a [`*mut MaybeUninit`](MaybeUninit) to the per-CPU variable on the CPU represented + /// by `cpu`. Note that without some kind of synchronization, use of the returned pointer may + /// cause a data race. It is the caller's responsibility to use the returned pointer in a + /// reasonable way. + /// + /// # Returns + /// - The returned pointer is valid only if `self` is (that is, it points to a live allocation + /// correctly sized and aligned to hold a `T`) + /// - The returned pointer is valid only if the bit corresponding to `cpu` is set in + /// [`kernel::cpumask::Cpumask::possible_cpus()`]. + pub fn get_remote_ptr(&self, cpu: CpuId) -> *mut MaybeUninit { + // SAFETY: `bindings::per_cpu_ptr` is just doing pointer arithmetic. The returned pointer + // may not be valid (under the conditions specified in this function's documentation), but + // the act of producing the pointer is safe. + unsafe { bindings::per_cpu_ptr(self.0.cast(), cpu.as_u32()) }.cast() + } } // SAFETY: Sending a [`PerCpuPtr`] to another thread is safe because as soon as it's sent, the @@ -146,9 +168,9 @@ impl Copy for PerCpuPtr {} /// A trait representing a per-CPU variable. /// -/// This is implemented for [`StaticPerCpu`]. The main usage of this trait is to call -/// [`Self::get_mut`] to get a [`PerCpuToken`] that can be used to access the underlying per-CPU -/// variable. +/// This is implemented for both [`StaticPerCpu`] and [`DynamicPerCpu`]. The main usage of +/// this trait is to call [`Self::get_mut`] to get a [`PerCpuToken`] that can be used to access the +/// underlying per-CPU variable. /// /// See [`PerCpuToken::with`]. pub trait PerCpu { diff --git a/rust/kernel/percpu/dynamic.rs b/rust/kernel/percpu/dynamic.rs new file mode 100644 index 000000000000..1863f31a2817 --- /dev/null +++ b/rust/kernel/percpu/dynamic.rs @@ -0,0 +1,130 @@ +// SPDX-License-Identifier: GPL-2.0 +//! Dynamically allocated per-CPU variables. + +use super::*; + +use crate::alloc::Flags; +use crate::bindings::{alloc_percpu, free_percpu}; +use crate::cpumask::Cpumask; +use crate::prelude::*; +use crate::sync::Arc; +use core::mem::{align_of, size_of, MaybeUninit}; + +/// Represents a dynamic allocation of a per-CPU variable via `alloc_percpu`. Calls `free_percpu` +/// when dropped. +/// +/// # Contents +/// Note that the allocated memory need not be initialized, and this type does not track when/if +/// the memory location on any particular CPU has been initialized. This means that it cannot tell +/// whether it should drop the *contents* of the allocation when it is dropped. It is up to the +/// user to do this via something like [`core::ptr::drop_in_place`]. +pub struct PerCpuAllocation(PerCpuPtr); + +impl PerCpuAllocation { + /// Dynamically allocates a space in the per-CPU area suitably sized and aligned to hold a `T`, + /// initially filled with the zero value for `T`. + /// + /// Returns [`None`] under the same circumstances the C function `alloc_percpu` returns `NULL`. + pub fn new_zero() -> Option> { + let ptr: *mut MaybeUninit = + // SAFETY: No preconditions to call `alloc_percpu`; `MaybeUninit` is + // `#[repr(transparent)]`, so we can cast a `*mut T` to it. + unsafe { alloc_percpu(size_of::(), align_of::()) }.cast(); + if ptr.is_null() { + return None; + } + + // alloc_percpu returns zero'ed memory + Some(Self(PerCpuPtr::new(ptr))) + } +} + +impl PerCpuAllocation { + /// Makes a per-CPU allocation sized and aligned to hold a `T`. + /// + /// Returns [`None`] under the same circumstances the C function `alloc_percpu` returns `NULL`. + pub fn new_uninit() -> Option> { + let ptr: *mut MaybeUninit = + // SAFETY: No preconditions to call `alloc_percpu`; `MaybeUninit` is + // `#[repr(transparent)]`, so we can cast a `*mut T` to it. + unsafe { alloc_percpu(size_of::(), align_of::()) }.cast(); + if ptr.is_null() { + return None; + } + + Some(Self(PerCpuPtr::new(ptr))) + } +} + +impl Drop for PerCpuAllocation { + fn drop(&mut self) { + // SAFETY: self.0.0 was returned by alloc_percpu, and so was a valid pointer into + // the percpu area, and has remained valid by the invariants of PerCpuAllocation. + unsafe { free_percpu(self.0 .0.cast()) } + } +} + +/// Holds a dynamically-allocated per-CPU variable. +#[derive(Clone)] +pub struct DynamicPerCpu { + // INVARIANT: `alloc` is `Some` unless this object is in the process of being dropped. + // INVARIANT: The allocation held by `alloc` is sized and aligned for a `T`. + // INVARIANT: The memory location in each CPU's per-CPU area pointed at by the alloc is + // initialized. + alloc: Option>>, +} + +impl DynamicPerCpu { + /// Allocates a new per-CPU variable + /// + /// # Arguments + /// * `flags` - [`Flags`] used to allocate an [`Arc`] that keeps track of the underlying + /// [`PerCpuAllocation`]. + pub fn new_zero(flags: Flags) -> Option { + let alloc: PerCpuAllocation = PerCpuAllocation::new_zero()?; + + let arc = Arc::new(alloc, flags).ok()?; + + Some(Self { alloc: Some(arc) }) + } +} + +impl PerCpu for DynamicPerCpu { + unsafe fn get_mut(&mut self, guard: CpuGuard) -> PerCpuToken<'_, T> { + // SAFETY: + // 1. Invariants of this type assure that `alloc` is `Some`. + // 2. The requirements of `PerCpu::get_mut` ensure that no other `[Checked]PerCpuToken` + // exists on the current CPU. + // 3. The invariants of `DynamicPerCpu` ensure that the contents of the allocation are + // initialized on each CPU. + // 4. The existence of a reference to the `PerCpuAllocation` ensures that the allocation is + // live. + // 5. The invariants of `DynamicPerCpu` ensure that the allocation is sized and aligned for + // a `T`. + unsafe { PerCpuToken::new(guard, &self.alloc.as_ref().unwrap_unchecked().0) } + } +} + +impl Drop for DynamicPerCpu { + fn drop(&mut self) { + // SAFETY: This type's invariant ensures that `self.alloc` is `Some`. + let alloc = unsafe { self.alloc.take().unwrap_unchecked() }; + if let Some(unique_alloc) = alloc.into_unique_or_drop() { + let ptr = unique_alloc.0; + for cpu in Cpumask::possible_cpus().iter() { + let remote_ptr = ptr.get_remote_ptr(cpu); + // SAFETY: `remote_ptr` is valid because the allocation it points to is still live, + // `cpu` appears in `Cpumask::possible_cpus()`, and the original allocation was + // sized and aligned for a `T`. + // + // This type's invariant ensures that the memory location in each CPU's per-CPU + // area pointed at by `alloc.0` has been initialized. We have a `UniqueArc`, so we + // know we're the only ones with a reference to the memory. These two facts + // together satisfy the requirements for `assume_init_drop`. + unsafe { + (*remote_ptr).assume_init_drop(); + } + } + } + } +} -- 2.34.1