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//! A heap-less, interrupt-safe, lock-free memory pool (\*) //! //! (\*) Currently, the implementation is only lock-free *and* `Sync` on ARMv7-M devices //! //! # Examples //! //! The most common way of using this pool is as a global singleton; the singleton mode gives you //! automatic deallocation of memory blocks on `drop`. //! //! ``` ignore //! #![no_main] //! #![no_std] //! //! use heapless::{pool, pool::singleton::Box}; //! //! // instantiate a memory pool of `[u8; 128]` blocks as a global singleton //! pool!(A: [u8; 128]); //! //! #[entry] //! fn main() -> ! { //! static mut MEMORY: [u8; 1024] = [0; 1024]; //! //! // increase the capacity of the pool by ~8 blocks //! A::grow(MEMORY); //! //! // claim a block of memory //! // note that the type is `Box<A>`, and not `Box<[u8; 128]>` //! // `A` is the "name" of the pool //! let x: Box<A, _> = A::alloc().unwrap(); //! loop { //! // .. do stuff with `x` .. //! } //! } //! //! #[exception] //! fn SysTick() { //! // claim a block of memory //! let y = A::alloc().unwrap(); //! //! // .. do stuff with `y` .. //! //! // return the memory block to the pool //! drop(y); //! } //! ``` //! //! # Portability //! //! This pool internally uses a Treiber stack which is known to be susceptible to the ABA problem. //! The only counter measure against the ABA problem that this implementation currently takes is //! relying on LL/SC (Link-local / Store-conditional) instructions being used to implement CAS loops //! on the target architecture (see section on ['Soundness'](#soundness) for more information). For //! this reason, `Pool` only implements `Sync` when compiling for ARMv7-M. //! //! Also note that ARMv6-M lacks the primitives for CAS loops so this module does *not* exist for //! `thumbv6m-none-eabi`. //! //! # Soundness //! //! This pool uses a Treiber stack to keep a list of free memory blocks (nodes). Each of these //! nodes has a pointer to the next node. To claim a memory block we simply pop a node from the //! top of the stack and use it as a memory block. The pop operation consists of swapping the //! current head (top) node with the node below it. The Rust code for the `pop` operation is shown //! below: //! //! ``` ignore //! fn pop(&self) -> Option<NonNull<Node<T>>> { //! let fetch_order = ..; //! let set_order = ..; //! //! // `self.head` has type `AtomicPtr<Node<T>>` //! let mut head = self.head.load(fetch_order); //! loop { //! if let Some(nn_head) = NonNull::new(head) { //! let next = unsafe { (*head).next }; //! //! // <~ preempted //! //! match self //! .head //! .compare_exchange_weak(head, next, set_order, fetch_order) //! { //! Ok(_) => break Some(nn_head), //! // head was changed by some interrupt handler / thread //! Err(new_head) => head = new_head, //! } //! } else { //! // stack is observed as empty //! break None; //! } //! } //! } //! ``` //! //! In general, the `pop` operation is susceptible to the ABA problem. If this operation gets //! preempted by some interrupt handler somewhere between the `head.load` and the //! `compare_and_exchange_weak`, and that handler modifies the stack in such a way that the head //! (top) of the stack remains unchanged then resuming the `pop` operation will corrupt the stack. //! //! An example: imagine we are doing on `pop` on stack that contains these nodes: `A -> B -> C`, //! `A` is the head (top), `B` is next to `A` and `C` is next to `B`. The `pop` operation will do a //! `CAS(&self.head, A, B)` operation to atomically change the head to `B` iff it currently is `A`. //! Now, let's say a handler preempts the `pop` operation before the `CAS` operation starts and it //! `pop`s the stack twice and then `push`es back the `A` node; now the state of the stack is `A -> //! C`. When the original `pop` operation is resumed it will succeed in doing the `CAS` operation //! setting `B` as the head of the stack. However, `B` was used by the handler as a memory block and //! no longer is a valid free node. As a result the stack, and thus the allocator, is in a invalid //! state. //! //! However, not all is lost because Cortex-M devices use LL/SC (Link-local / Store-conditional) //! operations to implement CAS loops. Let's look at the actual disassembly of `pop`. //! //! ``` text //! 08000130 <<heapless::pool::Pool<T>>::pop>: //! 8000130: 6802 ldr r2, [r0, #0] //! 8000132: e00c b.n 800014e <<heapless::pool::Pool<T>>::pop+0x1e> //! 8000134: 4611 mov r1, r2 //! 8000136: f8d2 c000 ldr.w ip, [r2] //! 800013a: e850 2f00 ldrex r2, [r0] //! 800013e: 428a cmp r2, r1 //! 8000140: d103 bne.n 800014a <<heapless::pool::Pool<T>>::pop+0x1a> //! 8000142: e840 c300 strex r3, ip, [r0] //! 8000146: b913 cbnz r3, 800014e <<heapless::pool::Pool<T>>::pop+0x1e> //! 8000148: e004 b.n 8000154 <<heapless::pool::Pool<T>>::pop+0x24> //! 800014a: f3bf 8f2f clrex //! 800014e: 2a00 cmp r2, #0 //! 8000150: d1f0 bne.n 8000134 <<heapless::pool::Pool<T>>::pop+0x4> //! 8000152: 2100 movs r1, #0 //! 8000154: 4608 mov r0, r1 //! 8000156: 4770 bx lr //! ``` //! //! LDREX ("load exclusive") is the LL instruction, and STREX ("store exclusive") is the SC //! instruction (see [1](#references)). On the Cortex-M, STREX will always fail if the processor //! takes an exception between it and its corresponding LDREX operation (see [2](#references)). If //! STREX fails then the CAS loop is retried (see instruction @ `0x8000146`). On single core //! systems, preemption is required to run into the ABA problem and on Cortex-M devices preemption //! always involves taking an exception. Thus the underlying LL/SC operations prevent the ABA //! problem on Cortex-M. //! //! # References //! //! 1. [Cortex-M3 Devices Generic User Guide (DUI 0552A)][0], Section 2.2.7 "Synchronization //! primitives" //! //! [0]: http://infocenter.arm.com/help/topic/com.arm.doc.dui0552a/DUI0552A_cortex_m3_dgug.pdf //! //! 2. [ARMv7-M Architecture Reference Manual (DDI 0403E.b)][1], Section A3.4 "Synchronization and //! semaphores" //! //! [1]: https://static.docs.arm.com/ddi0403/eb/DDI0403E_B_armv7m_arm.pdf use core::{ cell::UnsafeCell, fmt, marker::PhantomData, ops::{Deref, DerefMut}, ptr::{self, NonNull}, sync::atomic::AtomicPtr, }; #[cfg(not(armv6m))] use core::{any::TypeId, mem, sync::atomic::Ordering}; use as_slice::{AsMutSlice, AsSlice}; pub mod singleton; /// A lock-free memory pool pub struct Pool<T> { head: AtomicPtr<Node<T>>, // Current implementation is unsound on architectures that don't have LL/SC semantics so this // struct is not `Sync` on those platforms _not_send_or_sync: PhantomData<*const ()>, } // NOTE: Here we lie about `Pool` implementing `Sync` on x86_64. This is not true but it lets us // test the `pool!` and `singleton::Pool` abstractions. We just have to be careful not to use the // pool in a multi-threaded context #[cfg(any(armv7m, test))] unsafe impl<T> Sync for Pool<T> {} unsafe impl<T> Send for Pool<T> {} impl<T> Pool<T> { min_const_fn! { /// Creates a new empty pool pub const fn new() -> Self { Pool { head: AtomicPtr::new(ptr::null_mut()), _not_send_or_sync: PhantomData, } } } /// Claims a memory block from the pool /// /// Returns `None` when the pool is observed as exhausted /// /// *NOTE:* This method does *not* have bounded execution time because it contains a CAS loop pub fn alloc(&self) -> Option<Box<T, Uninit>> { if let Some(node) = self.pop() { Some(Box { node, _state: PhantomData, }) } else { None } } /// Returns a memory block to the pool /// /// *NOTE*: `T`'s destructor (if any) will run on `value` iff `S = Init` /// /// *NOTE:* This method does *not* have bounded execution time because it contains a CAS loop pub fn free<S>(&self, value: Box<T, S>) where S: 'static, { if TypeId::of::<S>() == TypeId::of::<Init>() { unsafe { ptr::drop_in_place(value.node.as_ref().data.get()); } } self.push(value.node) } /// Increases the capacity of the pool /// /// This method might *not* fully utilize the given memory block due to alignment requirements. /// /// This method returns the number of *new* blocks that can be allocated. pub fn grow(&self, memory: &'static mut [u8]) -> usize { let mut p = memory.as_mut_ptr(); let mut len = memory.len(); let align = mem::align_of::<Node<T>>(); let sz = mem::size_of::<Node<T>>(); let rem = (p as usize) % align; if rem != 0 { let offset = align - rem; if offset >= len { // slice is too small return 0; } p = unsafe { p.add(offset) }; len -= offset; } let mut n = 0; while len >= sz { self.push(unsafe { NonNull::new_unchecked(p as *mut _) }); n += 1; p = unsafe { p.add(sz) }; len -= sz; } n } fn pop(&self) -> Option<NonNull<Node<T>>> { // NOTE `Ordering`s come from crossbeam's (v0.6.0) `TreiberStack` loop { let head = self.head.load(Ordering::Acquire); if let Some(nn_head) = NonNull::new(head) { let next = unsafe { (*head).next }; match self.head.compare_exchange_weak( head, next, Ordering::Release, // success Ordering::Relaxed, // failure ) { Ok(_) => break Some(nn_head), // head was changed by some interrupt handler Err(_) => continue, } } else { // stack is observed as empty break None; } } } fn push(&self, mut new_head: NonNull<Node<T>>) { // NOTE `Ordering`s come from crossbeam's (v0.6.0) `TreiberStack` let mut head = self.head.load(Ordering::Relaxed); loop { unsafe { new_head.as_mut().next = head } match self.head.compare_exchange_weak( head, new_head.as_ptr(), Ordering::Release, // success Ordering::Relaxed, // failure ) { Ok(_) => return, // head changed Err(p) => head = p, } } } } struct Node<T> { data: UnsafeCell<T>, next: *mut Node<T>, } /// A memory block pub struct Box<T, STATE = Init> { _state: PhantomData<STATE>, node: NonNull<Node<T>>, } impl<T> Box<T, Uninit> { /// Initializes this memory block pub fn init(self, val: T) -> Box<T, Init> { unsafe { ptr::write(self.node.as_ref().data.get(), val); } Box { node: self.node, _state: PhantomData, } } } /// Uninitialized type state pub enum Uninit {} /// Initialized type state pub enum Init {} unsafe impl<T, S> Send for Box<T, S> where T: Send {} unsafe impl<T, S> Sync for Box<T, S> where T: Sync {} impl<A> AsSlice for Box<A> where A: AsSlice, { type Element = A::Element; fn as_slice(&self) -> &[A::Element] { self.deref().as_slice() } } impl<A> AsMutSlice for Box<A> where A: AsMutSlice, { fn as_mut_slice(&mut self) -> &mut [A::Element] { self.deref_mut().as_mut_slice() } } impl<T> Deref for Box<T> { type Target = T; fn deref(&self) -> &T { unsafe { &*self.node.as_ref().data.get() } } } impl<T> DerefMut for Box<T> { fn deref_mut(&mut self) -> &mut T { unsafe { &mut *self.node.as_ref().data.get() } } } impl<T> fmt::Debug for Box<T> where T: fmt::Debug, { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { <T as fmt::Debug>::fmt(self, f) } } impl<T> fmt::Display for Box<T> where T: fmt::Display, { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { <T as fmt::Display>::fmt(self, f) } } #[cfg(test)] mod tests { use core::{ mem, sync::atomic::{AtomicUsize, Ordering}, }; use super::Pool; #[test] fn grow() { static mut MEMORY: [u8; 1024] = [0; 1024]; #[cfg(feature = "min-const-fn")] static POOL: Pool<[u8; 128]> = Pool::new(); #[allow(non_snake_case)] #[cfg(not(feature = "min-const-fn"))] let POOL: Pool<[u8; 128]> = Pool::new(); unsafe { POOL.grow(&mut MEMORY); } for _ in 0..7 { assert!(POOL.alloc().is_some()); } } #[test] fn sanity() { static mut MEMORY: [u8; 31] = [0; 31]; #[cfg(feature = "min-const-fn")] static POOL: Pool<u8> = Pool::new(); #[allow(non_snake_case)] #[cfg(not(feature = "min-const-fn"))] let POOL: Pool<u8> = Pool::new(); // empty pool assert!(POOL.alloc().is_none()); POOL.grow(unsafe { &mut MEMORY }); let x = POOL.alloc().unwrap().init(0); assert_eq!(*x, 0); // pool exhausted assert!(POOL.alloc().is_none()); POOL.free(x); // should be possible to allocate again assert_eq!(*POOL.alloc().unwrap().init(1), 1); } #[test] fn destructors() { static COUNT: AtomicUsize = AtomicUsize::new(0); struct X; impl X { fn new() -> X { COUNT.fetch_add(1, Ordering::Relaxed); X } } impl Drop for X { fn drop(&mut self) { COUNT.fetch_sub(1, Ordering::Relaxed); } } static mut MEMORY: [u8; 31] = [0; 31]; #[cfg(feature = "min-const-fn")] static POOL: Pool<X> = Pool::new(); #[allow(non_snake_case)] #[cfg(not(feature = "min-const-fn"))] let POOL: Pool<X> = Pool::new(); POOL.grow(unsafe { &mut MEMORY }); let x = POOL.alloc().unwrap().init(X::new()); let y = POOL.alloc().unwrap().init(X::new()); let z = POOL.alloc().unwrap().init(X::new()); assert_eq!(COUNT.load(Ordering::Relaxed), 3); // this leaks memory drop(x); assert_eq!(COUNT.load(Ordering::Relaxed), 3); // this leaks memory mem::forget(y); assert_eq!(COUNT.load(Ordering::Relaxed), 3); // this runs `X` destructor POOL.free(z); assert_eq!(COUNT.load(Ordering::Relaxed), 2); } }