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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 is not aware that the underlying Bitmap abstraction handles lengths below BITS_PER_LONG without allocation. Suggested-by: Alice Ryhl Suggested-by: Yury Norov Signed-off-by: Burak Emir --- MAINTAINERS | 1 + rust/kernel/id_pool.rs | 201 +++++++++++++++++++++++++++++++++++++++++ rust/kernel/lib.rs | 1 + 3 files changed, 203 insertions(+) create mode 100644 rust/kernel/id_pool.rs diff --git a/MAINTAINERS b/MAINTAINERS index d448b73c5934..161281251108 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -4142,6 +4142,7 @@ R: Yury Norov 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 diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs new file mode 100644 index 000000000000..8f07526bb580 --- /dev/null +++ b/rust/kernel/id_pool.rs @@ -0,0 +1,201 @@ +// 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 zero bits in a bitmap. +/// +/// The ID pool can grow or shrink as needed. It has been designed +/// to support the scenario where users need to control the time +/// of allocation of a new backing bitmap, which may require release +/// of locks. +/// These operations then, are 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_alloc().alloc(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_alloc(guarded_pool: &SpinLock) -> Result { +/// let mut pool = guarded_pool.lock(); +/// loop { +/// match pool.acquire_next_id(0) { +/// Some(index) => return Ok(index), +/// None => { +/// let alloc_request = pool.grow_alloc(); +/// drop(pool); +/// let resizer = alloc_request.alloc(GFP_KERNEL)?; +/// pool = guarded_pool.lock(); +/// pool.grow(resizer) +/// } +/// } +/// } +/// } +/// ``` +pub struct IdPool { + map: Bitmap, +} + +/// Returned when the `IdPool` should change size. +pub struct AllocRequest { + nbits: usize, +} + +/// Contains an allocated `Bitmap` for resizing `IdPool`. +pub struct PoolResizer { + new: Bitmap, +} + +impl AllocRequest { + /// Allocates a new `Bitmap` for `IdPool`. + pub fn alloc(&self, flags: Flags) -> Result { + let new = Bitmap::new(self.nbits, flags)?; + Ok(PoolResizer { new }) + } +} + +impl IdPool { + /// Constructs a new `[IdPool]`. + #[inline] + pub fn new(nbits: usize, flags: Flags) -> Result { + let map = Bitmap::new(nbits, flags)?; + Ok(Self { map }) + } + + /// Returns how many IDs this pool can currently have. + #[inline] + pub fn len(&self) -> usize { + self.map.len() + } + + /// Returns an [`AllocRequest`] if the [`IdPool`] can be shrunk, [`None`] otherwise. + /// + /// # Examples + /// + /// ``` + /// use kernel::alloc::{AllocError, flags::GFP_KERNEL}; + /// use kernel::id_pool::{AllocRequest, IdPool}; + /// + /// let mut pool = IdPool::new(1024, GFP_KERNEL)?; + /// let alloc_request = pool.shrink_alloc().ok_or(AllocError)?; + /// let resizer = alloc_request.alloc(GFP_KERNEL)?; + /// pool.shrink(resizer); + /// assert_eq!(pool.len(), kernel::bindings::BITS_PER_LONG as usize); + /// # Ok::<(), AllocError>(()) + /// ``` + #[inline] + pub fn shrink_alloc(&self) -> Option { + let len = self.map.len(); + 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. + */ + match self.map.last_bit() { + Some(bit) => { + if bit < (len >> 2) { + Some(AllocRequest { nbits: len >> 1 }) + } else { + None + } + } + None => Some(AllocRequest { + nbits: bindings::BITS_PER_LONG as usize, + }), + } + } + + /// Shrinks pool by using a new `Bitmap`, if still possible. + #[inline] + pub fn shrink(&mut self, mut resizer: PoolResizer) { + // Verify that shrinking is still possible. The `resizer` + // bitmap might have been allocated without locks, so this call + // could now be outdated. In this case, drop `resizer` and move on. + if let Some(AllocRequest { nbits }) = self.shrink_alloc() { + if nbits <= resizer.new.len() { + resizer.new.copy_and_extend(&self.map); + self.map = resizer.new; + return; + } + } + } + + /// Returns an `AllocRequest` for growing this `IdPool`. + #[inline] + pub fn grow_alloc(&self) -> AllocRequest { + AllocRequest { + nbits: self.map.len() << 1, + } + } + + /// Grows pool by using a new `Bitmap`, if still necessary. + #[inline] + pub fn grow(&mut self, mut resizer: PoolResizer) { + // `resizer` bitmap might have been allocated without locks, + // so this call could now be outdated. In this case, 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_alloc` is needed. + #[inline] + pub fn acquire_next_id(&mut self, offset: usize) -> Option { + match self.map.next_zero_bit(offset) { + res @ Some(nr) => { + self.map.set_bit(nr); + res + } + None => None, + } + } + + /// 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 94eb150c52c7..4c6b45dcad04 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; -- 2.49.0.805.g082f7c87e0-goog