This is a port of the Binder data structure introduced in commit
15d9da3f818c ("binder: use bitmap for faster descriptor lookup") to
Rust.Like drivers/android/dbitmap.h, the ID pool abstraction lets clients acquire and release IDs. The implementation uses a bitmap to know what IDs are in use, and gives clients fine-grained control over the time of allocation. This fine-grained control is needed in the Android Binder. We provide an example that release a spinlock for allocation and unit tests (rustdoc examples). The implementation does not permit shrinking below capacity below BITS_PER_LONG. Suggested-by: Alice Ryhl <[email protected]> Suggested-by: Yury Norov <[email protected]> Signed-off-by: Burak Emir <[email protected]> --- MAINTAINERS | 1 + rust/kernel/id_pool.rs | 222 +++++++++++++++++++++++++++++++++++++++++ rust/kernel/lib.rs | 1 + 3 files changed, 224 insertions(+) create mode 100644 rust/kernel/id_pool.rs diff --git a/MAINTAINERS b/MAINTAINERS index 943d85ed1876..bc95d98f266b 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -4134,6 +4134,7 @@ R: Yury Norov <[email protected]> S: Maintained F: lib/find_bit_benchmark_rust.rs F: rust/kernel/bitmap.rs +F: rust/kernel/id_pool.rs BITOPS API M: Yury Norov <[email protected]> diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs new file mode 100644 index 000000000000..cf26d405d9bb --- /dev/null +++ b/rust/kernel/id_pool.rs @@ -0,0 +1,222 @@ +// SPDX-License-Identifier: GPL-2.0 + +// Copyright (C) 2025 Google LLC. + +//! Rust API for an ID pool backed by a [`Bitmap`]. + +use crate::alloc::{AllocError, Flags}; +use crate::bitmap::Bitmap; + +/// Represents a dynamic ID pool backed by a [`Bitmap`]. +/// +/// Clients acquire and release IDs from unset bits in a bitmap. +/// +/// The capacity of the ID pool may be adjusted by users as +/// needed. The API supports the scenario where users need precise control +/// over the time of allocation of a new backing bitmap, which may require +/// release of spinlock. +/// Due to concurrent updates, all operations are re-verified to determine +/// if the grow or shrink is sill valid. +/// +/// # Examples +/// +/// Basic usage +/// +/// ``` +/// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; +/// use kernel::id_pool::IdPool; +/// +/// let mut pool = IdPool::new(64, GFP_KERNEL)?; +/// for i in 0..64 { +/// assert_eq!(i, pool.acquire_next_id(i).ok_or(ENOSPC)?); +/// } +/// +/// pool.release_id(23); +/// assert_eq!(23, pool.acquire_next_id(0).ok_or(ENOSPC)?); +/// +/// assert_eq!(None, pool.acquire_next_id(0)); // time to realloc. +/// let resizer = pool.grow_request().ok_or(ENOSPC)?.realloc(GFP_KERNEL)?; +/// pool.grow(resizer); +/// +/// assert_eq!(pool.acquire_next_id(0), Some(64)); +/// # Ok::<(), Error>(()) +/// ``` +/// +/// Releasing spinlock to grow the pool +/// +/// ```no_run +/// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; +/// use kernel::sync::{new_spinlock, SpinLock}; +/// use kernel::id_pool::IdPool; +/// +/// fn get_id_maybe_realloc(guarded_pool: &SpinLock<IdPool>) -> Result<usize, AllocError> { +/// let mut pool = guarded_pool.lock(); +/// loop { +/// match pool.acquire_next_id(0) { +/// Some(index) => return Ok(index), +/// None => { +/// let alloc_request = pool.grow_request(); +/// drop(pool); +/// let resizer = alloc_request.ok_or(AllocError)?.realloc(GFP_KERNEL)?; +/// pool = guarded_pool.lock(); +/// pool.grow(resizer) +/// } +/// } +/// } +/// } +/// ``` +pub struct IdPool { + map: Bitmap, +} + +/// Indicates that an [`IdPool`] should change to a new target size. +pub struct ReallocRequest { + num_ids: usize, +} + +/// Contains a [`Bitmap`] of a size suitable for reallocating [`IdPool`]. +pub struct PoolResizer { + new: Bitmap, +} + +impl ReallocRequest { + /// Allocates a new backing [`Bitmap`] for [`IdPool`]. + /// + /// This method only prepares reallocation and does not complete it. + /// Reallocation will complete after passing the [`PoolResizer`] to the + /// [`IdPool::grow`] or [`IdPool::shrink`] operation, which will check + /// that reallocation still makes sense. + pub fn realloc(&self, flags: Flags) -> Result<PoolResizer, AllocError> { + let new = Bitmap::new(self.num_ids, flags)?; + Ok(PoolResizer { new }) + } +} + +impl IdPool { + /// Constructs a new `[IdPool]`. + /// + /// A capacity below [`BITS_PER_LONG`] is adjusted to [`BITS_PER_LONG`]. + #[inline] + pub fn new(num_ids: usize, flags: Flags) -> Result<Self, AllocError> { + let num_ids = core::cmp::max(num_ids, bindings::BITS_PER_LONG as usize); + let map = Bitmap::new(num_ids, flags)?; + Ok(Self { map }) + } + + /// Returns how many IDs this pool can currently have. + #[inline] + pub fn len(&self) -> usize { + self.map.len() + } + + /// Returns a [`ReallocRequest`] if the [`IdPool`] can be shrunk, [`None`] otherwise. + /// + /// The capacity of an [`IdPool`] cannot be shrunk below [`BITS_PER_LONG`]. + /// + /// # Examples + /// + /// ``` + /// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; + /// use kernel::id_pool::{ReallocRequest, IdPool}; + /// + /// let mut pool = IdPool::new(1024, GFP_KERNEL)?; + /// let alloc_request = pool.shrink_request().ok_or(AllocError)?; + /// let resizer = alloc_request.realloc(GFP_KERNEL)?; + /// pool.shrink(resizer); + /// assert_eq!(pool.len(), kernel::bindings::BITS_PER_LONG as usize); + /// # Ok::<(), AllocError>(()) + /// ``` + #[inline] + pub fn shrink_request(&self) -> Option<ReallocRequest> { + let len = self.map.len(); + // Shrinking below [`BITS_PER_LONG`] is never possible. + if len <= bindings::BITS_PER_LONG as usize { + return None; + } + // Determine if the bitmap can shrink based on the position of + // its last set bit. If the bit is within the first quarter of + // the bitmap then shrinking is possible. In this case, the + // bitmap should shrink to half its current size. + let last_bit = self.map.last_bit(); + if last_bit.is_none() { + return Some(ReallocRequest { + num_ids: bindings::BITS_PER_LONG as usize, + }); + } + let bit = last_bit.unwrap(); + if bit >= (len / 4) { + return None; + } + let num_ids = core::cmp::max(bindings::BITS_PER_LONG as usize, len / 2); + Some(ReallocRequest { num_ids }) + } + + /// Shrinks pool by using a new [`Bitmap`], if still possible. + #[inline] + pub fn shrink(&mut self, mut resizer: PoolResizer) { + // Between request to shrink that led to allocation of `resizer` and now, + // bits may have changed. + // Verify that shrinking is still possible. In case shrinking to + // the size of `resizer` is no longer possible, do nothing, + // drop `resizer` and move on. + let updated = self.shrink_request(); + if updated.is_none() { + return; + } + if updated.unwrap().num_ids > resizer.new.len() { + return; + } + + resizer.new.copy_and_extend(&self.map); + self.map = resizer.new; + } + + /// Returns a [`ReallocRequest`] for growing this [`IdPool`], if possible. + /// + /// The capacity of an [`IdPool`] cannot be grown above [`i32::MAX`]. + #[inline] + pub fn grow_request(&self) -> Option<ReallocRequest> { + let num_ids = self.map.len() * 2; + if num_ids > i32::MAX.try_into().unwrap() { + return None; + } + Some(ReallocRequest { num_ids }) + } + + /// Grows pool by using a new [`Bitmap`], if still necessary. + /// + /// The `resizer` arguments has to be obtained by calling [`grow_request`] + /// on this object and performing a `realloc`. + #[inline] + pub fn grow(&mut self, mut resizer: PoolResizer) { + // Between request to grow that led to allocation of `resizer` and now, + // another thread may have already grown the capacity. + // In this case, do nothing, drop `resizer` and move on. + if resizer.new.len() <= self.map.len() { + return; + } + + resizer.new.copy_and_extend(&self.map); + self.map = resizer.new; + } + + /// Acquires a new ID by finding and setting the next zero bit in the + /// bitmap. + /// + /// Upon success, returns its index. Otherwise, returns `None` + /// to indicate that a `grow_request` is needed. + #[inline] + pub fn acquire_next_id(&mut self, offset: usize) -> Option<usize> { + let next_zero_bit = self.map.next_zero_bit(offset); + if let Some(nr) = next_zero_bit { + self.map.set_bit(nr); + } + next_zero_bit + } + + /// Releases an ID. + #[inline] + pub fn release_id(&mut self, id: usize) { + self.map.clear_bit(id); + } +} diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 8c4161cd82ac..d7def807900a 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -54,6 +54,7 @@ #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)] pub mod firmware; pub mod fs; +pub mod id_pool; pub mod init; pub mod io; pub mod ioctl; -- 2.49.0.1204.g71687c7c1d-goog
