From 5bd0da77407f51c7b8ccd11c6e852a166fda4df1 Mon Sep 17 00:00:00 2001 From: Brian Cully Date: Sun, 4 Aug 2019 16:18:25 -0400 Subject: Initial commit. --- .gitignore | 3 + COPYING | 674 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ COPYING.LESSER | 165 ++++++++++++++ Cargo.toml | 9 + README.md | 4 + src/lib.rs | 221 +++++++++++++++++++ 6 files changed, 1076 insertions(+) create mode 100644 .gitignore create mode 100644 COPYING create mode 100644 COPYING.LESSER create mode 100644 Cargo.toml create mode 100644 README.md create mode 100755 src/lib.rs diff --git a/.gitignore b/.gitignore new file mode 100644 index 0000000..1d8e8c5 --- /dev/null +++ b/.gitignore @@ -0,0 +1,3 @@ +*/target +**/*.rs.bk +Cargo.lock diff --git a/COPYING b/COPYING new file mode 100644 index 0000000..f288702 --- /dev/null +++ b/COPYING @@ -0,0 +1,674 @@ + GNU GENERAL PUBLIC LICENSE + Version 3, 29 June 2007 + + Copyright (C) 2007 Free Software Foundation, Inc. + Everyone is permitted to copy and distribute verbatim copies + of this license document, but changing it is not allowed. + + Preamble + + The GNU General Public License is a free, copyleft license for +software and other kinds of works. + + The licenses for most software and other practical works are designed +to take away your freedom to share and change the works. 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If the Library as you +received it does not specify a version number of the GNU Lesser +General Public License, you may choose any version of the GNU Lesser +General Public License ever published by the Free Software Foundation. + + If the Library as you received it specifies that a proxy can decide +whether future versions of the GNU Lesser General Public License shall +apply, that proxy's public statement of acceptance of any version is +permanent authorization for you to choose that version for the +Library. diff --git a/Cargo.toml b/Cargo.toml new file mode 100644 index 0000000..66b8c10 --- /dev/null +++ b/Cargo.toml @@ -0,0 +1,9 @@ +[package] +name = "starb" +description = "STAtic Ring Buffers." +version = "0.1.0" +authors = ["Brian Cully "] +edition = "2018" +license = "LGPL-3.0-or-later" +repository = "https://github.com/bjc/starb.git" +readme = "README.md" diff --git a/README.md b/README.md new file mode 100644 index 0000000..c5d3207 --- /dev/null +++ b/README.md @@ -0,0 +1,4 @@ +# STAtic Ring Buffers + +This is a ring buffer implementation suitable for use in `no-std` +environments. diff --git a/src/lib.rs b/src/lib.rs new file mode 100755 index 0000000..23f2947 --- /dev/null +++ b/src/lib.rs @@ -0,0 +1,221 @@ +// FIXME: Currently RingBuffer requires its type parameter to be +// `Copy` because items are stored in a fixed-length array (which, +// itself, is required to allow creating RingBuffers +// statically). There may be work-arounds with ptr routines, and it +// should be investigated. +// +// TODO: Needs impls for Iter/IterMut. +#![no_std] +#![feature(const_fn)] + +use core::{ + cell::UnsafeCell, + marker::PhantomData, + sync::atomic::{AtomicUsize, Ordering}, +}; + +/// Underlying buffer capacity. Needs to be hard-coded for now, +/// because the size of the structure needs to be known at compile +/// time so it can be statically allocated or created on the stack. +/// +/// This will disappear when const generics appear. +const CAPACITY: usize = 1024; + +#[derive(Debug, PartialEq)] +pub enum Error { + BufferFull, +} + +/// A lock-free concurrent ring buffer that prevents overwriting data +/// before it is read. +pub struct RingBuffer { + head: AtomicUsize, + tail: AtomicUsize, + + // Backing store needs to be UnsafeCell to make sure it's not + // stored in read-only data sections. + buf: UnsafeCell<[T; CAPACITY]>, +} + +impl RingBuffer +where + T: Copy, +{ + /// Create a new RingBuffer. + pub const fn new(default: T) -> Self { + Self { + head: AtomicUsize::new(0), + tail: AtomicUsize::new(0), + buf: UnsafeCell::new([default; CAPACITY]), + } + } + + // Honestly, this should consume `self` or at least be `mut`, but + // it needs to be available in const static contexts, which + // prevents that. Basically all the rest of the unsafe stuff in + // here is a consequence of that. + // + // No, lazy_static is not an option, because it doesn't work on + // architectures where CAS atomics are missing. + pub const fn split<'a>(&'a self) -> (Reader, Writer) { + let rbr = Reader { + rb: self as *const _, + _marker: PhantomData, + }; + let rbw = Writer { + rb: self as *const _, + _marker: PhantomData, + }; + (rbr, rbw) + } +} + +pub struct Reader<'a, T> { + rb: *const RingBuffer, + _marker: PhantomData<&'a ()>, +} +unsafe impl Send for Reader<'_, T> where T: Send {} +unsafe impl Sync for Reader<'_, T> {} + +pub struct Writer<'a, T> { + rb: *const RingBuffer, + _marker: PhantomData<&'a ()>, +} +unsafe impl Send for Writer<'_, T> where T: Send {} +unsafe impl Sync for Writer<'_, T> {} + +impl<'a, T> Reader<'a, T> +where + T: Copy, +{ + pub fn len(&self) -> usize { + let rb: &mut RingBuffer = unsafe { core::mem::transmute(self.rb) }; + let h = rb.head.load(Ordering::SeqCst); + let t = rb.tail.load(Ordering::SeqCst); + let rc = (t + CAPACITY - h) % CAPACITY; + rc + } + + pub fn is_empty(&self) -> bool { + self.len() == 0 + } + + /// Returns the value at the start of the buffer and advances the + /// start of the buffer to the next element. + /// + /// If nothing is available in the buffer, returns `None` + pub fn shift(&mut self) -> Option { + let rb: &mut RingBuffer = unsafe { core::mem::transmute(self.rb) }; + let h = rb.head.load(Ordering::SeqCst); + let t = rb.tail.load(Ordering::SeqCst); + if h == t { + None + } else { + let nh = (h + 1) % CAPACITY; + let buf = unsafe { &mut *rb.buf.get() }; + let rc = Some(buf[h]); + rb.head.store(nh, Ordering::SeqCst); + rc + } + } +} + +impl<'a, T> Writer<'a, T> +where + T: Copy, +{ + /// Put `v` at the end of the buffer. + /// + /// Returns `BufferFull` if appending `v` would overlap with the + /// start of the buffer. + pub fn unshift(&mut self, v: T) -> Result<(), Error> { + let rb: &mut RingBuffer = unsafe { core::mem::transmute(self.rb) }; + let h = rb.head.load(Ordering::SeqCst); + let t = rb.tail.load(Ordering::SeqCst); + let nt = (t + 1) % CAPACITY; + // We can't allow overwrites of the head position, because it + // would then be possible to write to the same memory location + // that is being read. If reading a value of `T` takes more + // than one memory read, then reading from the head would + // produce garbage in this scenario. + if nt == h { + // FIXME: this comparison is wrong in the trivial + // 1-element buffer case which would never allow an + // `unshift`. In larger buffers it wastes a buffer slot. + Err(Error::BufferFull) + } else { + let buf = unsafe { &mut *rb.buf.get() }; + buf[t] = v; + rb.tail.store(nt, Ordering::SeqCst); + Ok(()) + } + } +} + +#[cfg(test)] +mod test { + use super::*; + + #[test] + fn detects_empty() { + let rb = RingBuffer::::new(false); + let (mut rbr, mut rbw) = rb.split(); + assert!(rbr.is_empty()); + rbw.unshift(true).ok(); + assert!(!rbr.is_empty()); + rbr.shift(); + assert!(rbr.is_empty()); + } + + #[test] + fn len_matches() { + let rb = RingBuffer::::new(false); + let (mut rbr, mut rbw) = rb.split(); + + // Count length up. + for i in 0..CAPACITY - 1 { + assert_eq!(rbr.len(), i); + assert_eq!(rbw.unshift(true), Ok(())); + } + + // ...and back down again. + for i in 0..CAPACITY - 1 { + assert_eq!(rbr.len(), CAPACITY - 1 - i); + rbr.shift(); + } + + // Final check for empty. + assert_eq!(rbr.len(), 0); + } + + #[test] + fn can_wrap() { + let rb = RingBuffer::::new(0); + let (mut rbr, mut rbw) = rb.split(); + + // Make sure we can store n-1 elements. + for i in 0..CAPACITY - 1 { + assert_eq!(rbw.unshift(i), Ok(())) + } + + // ...and that we can load them back again. + for i in 0..CAPACITY - 1 { + assert_eq!(rbr.shift(), Some(i)) + } + } + + #[test] + fn cannot_overwrite() { + let rb = RingBuffer::::new(0); + let (mut rbr, mut rbw) = rb.split(); + + for i in 0..CAPACITY - 1 { + assert_eq!(rbw.unshift(i), Ok(())); + } + assert_eq!(rbw.unshift(0xffff), Err(Error::BufferFull)); + + // We can drop an element to allow a slot to write to again. + rbr.shift(); + assert_eq!(rbw.unshift(0xffff), Ok(())); + } +} -- cgit v1.2.3