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//! This library provides a phase-fair reader-writer lock, as described in the
//! paper ["Reader-Writer Synchronization for Shared-Memory Multiprocessor
//! Real-Time Systems"](https://www.cs.unc.edu/~anderson/papers/ecrts09b.pdf)
//! by Brandenburg et. al.
//!
//! > Reader preference, writer preference, and task-fair reader-writer locks are
//! > shown to cause undue blocking in multiprocessor real-time systems. A new
//! > phase-fair reader-writer lock is proposed as an alternative that
//! > significantly reduces worst-case blocking for readers.
//!
//! # Example
//!
//! ```
//! use pflock::PFLock;
//!
//! let lock = PFLock::new(5);
//!
//! // many reader locks can be held at once
//! {
//! let r1 = lock.read();
//! let r2 = lock.read();
//! assert_eq!(*r1, 5);
//! assert_eq!(*r2, 5);
//! } // read locks are dropped at this point
//!
//! // only one write lock may be held, however
//! {
//! let mut w = lock.write();
//! *w += 1;
//! assert_eq!(*w, 6);
//! } // write lock is dropped here
//! ```
//!
//! # Spin vs. suspend
//!
//! `PFLock` is a spinlock specifically targeted at **short critical sections** and
//! does not suspend threads while blocking. Section 3 of the paper addresses this:
//!
//! > The terms “short” and “long” arise because (intuitively) spinning is
//! > appropriate only for short critical sections, since spinning wastes processor
//! > time. However, two recent studies have shown that, in terms of
//! > schedulability, spinning is usually preferable to suspending when overheads
//! > are considered [11, 15]. Based on these trends (and due to space
//! > constraints), we restrict our focus to short resources in this paper and
//! > delegate RW synchronization of long resources to future work.
#![no_std]
use core::hint::spin_loop;
use core::sync::atomic::{AtomicUsize, Ordering};
use lock_api::{GuardSend, RawRwLock, RwLock};
pub struct RawPFLock {
rin: AtomicUsize,
rout: AtomicUsize,
win: AtomicUsize,
wout: AtomicUsize,
}
const RINC: usize = 0x100; // reader increment
const WBITS: usize = 0x3; // writer bits in rin
const PRES: usize = 0x2; // writer present bit
const PHID: usize = 0x1; // phase ID bit
const ZERO_MASK: usize = !255usize;
unsafe impl RawRwLock for RawPFLock {
const INIT: RawPFLock = RawPFLock {
rin: AtomicUsize::new(0),
rout: AtomicUsize::new(0),
win: AtomicUsize::new(0),
wout: AtomicUsize::new(0),
};
type GuardMarker = GuardSend;
fn lock_shared(&self) {
// Increment the rin count and read the writer bits
let w = self.rin.fetch_add(RINC, Ordering::Relaxed) & WBITS;
// Spin (wait) if there is a writer present (w != 0), until either PRES
// and/or PHID flips
while (w != 0) && (w == (self.rin.load(Ordering::Relaxed) & WBITS)) {
spin_loop();
}
}
unsafe fn unlock_shared(&self) {
// Increment rout to mark the read-lock returned
self.rout.fetch_add(RINC, Ordering::Relaxed);
}
fn try_lock_shared(&self) -> bool {
let w = self.rin.fetch_add(RINC, Ordering::Relaxed) & WBITS;
if w == 0 || w != (self.rin.load(Ordering::Relaxed) & WBITS) {
true
} else {
self.rout.fetch_add(RINC, Ordering::Relaxed);
false
}
}
fn lock_exclusive(&self) {
// Wait until it is my turn to write-lock the resource
let wticket = self.win.fetch_add(1, Ordering::Relaxed);
while wticket != self.wout.load(Ordering::Relaxed) {
spin_loop();
}
// Set the write-bits of rin to indicate this writer is here
let w = PRES | (wticket & PHID);
let rticket = self.rin.fetch_add(w, Ordering::Relaxed);
// Wait until all current readers have finished (i.e. rout catches up)
while rticket != self.rout.load(Ordering::Relaxed) {
spin_loop();
}
}
unsafe fn unlock_exclusive(&self) {
// Clear the least-significant byte of rin
self.rin.fetch_and(ZERO_MASK, Ordering::Relaxed);
// Increment wout to indicate this write has released the lock
// Only one writer should ever be here
self.wout.fetch_add(1, Ordering::Relaxed);
}
fn try_lock_exclusive(&self) -> bool {
let wticket = self.win.fetch_add(1, Ordering::Relaxed);
if wticket != self.wout.load(Ordering::Relaxed) {
self.wout.fetch_add(1, Ordering::Relaxed);
return false;
}
let w = PRES | (wticket & PHID);
let rticket = self.rin.fetch_add(w, Ordering::Relaxed);
if rticket != self.rout.load(Ordering::Relaxed) {
self.rin.fetch_and(ZERO_MASK, Ordering::Relaxed);
self.wout.fetch_add(1, Ordering::Relaxed);
return false;
}
true
}
}
/// A phase-fair reader-writer lock.
pub type PFLock<T> = RwLock<RawPFLock, T>;
pub type PFLockGuard<'a, T> = lock_api::MutexGuard<'a, RawPFLock, T>;