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#![cfg_attr(not(feature = "rt"), allow(dead_code))] use crate::loom::cell::UnsafeCell; use crate::loom::sync::atomic::{AtomicBool, AtomicUsize}; use crate::loom::sync::{Arc, Mutex}; use crate::util::bit; use std::fmt; use std::mem; use std::ops; use std::ptr; use std::sync::atomic::Ordering::Relaxed; /// Amortized allocation for homogeneous data types. /// /// The slab pre-allocates chunks of memory to store values. It uses a similar /// growing strategy as `Vec`. When new capacity is needed, the slab grows by /// 2x. /// /// # Pages /// /// Unlike `Vec`, growing does not require moving existing elements. Instead of /// being a continuous chunk of memory for all elements, `Slab` is an array of /// arrays. The top-level array is an array of pages. Each page is 2x bigger /// than the previous one. When the slab grows, a new page is allocated. /// /// Pages are lazily initialized. /// /// # Allocating /// /// When allocating an object, first previously used slots are reused. If no /// previously used slot is available, a new slot is initialized in an existing /// page. If all pages are full, then a new page is allocated. /// /// When an allocated object is released, it is pushed into it's page's free /// list. Allocating scans all pages for a free slot. /// /// # Indexing /// /// The slab is able to index values using an address. Even when the indexed /// object has been released, it is still safe to index. This is a key ability /// for using the slab with the I/O driver. Addresses are registered with the /// OS's selector and I/O resources can be released without synchronizing with /// the OS. /// /// # Compaction /// /// `Slab::compact` will release pages that have been allocated but are no /// longer used. This is done by scanning the pages and finding pages with no /// allocated objects. These pages are then freed. /// /// # Synchronization /// /// The `Slab` structure is able to provide (mostly) unsynchronized reads to /// values stored in the slab. Insertions and removals are synchronized. Reading /// objects via `Ref` is fully unsynchronized. Indexing objects uses amortized /// synchronization. /// pub(crate) struct Slab<T> { /// Array of pages. Each page is synchronized. pages: [Arc<Page<T>>; NUM_PAGES], /// Caches the array pointer & number of initialized slots. cached: [CachedPage<T>; NUM_PAGES], } /// Allocate values in the associated slab. pub(crate) struct Allocator<T> { /// Pages in the slab. The first page has a capacity of 16 elements. Each /// following page has double the capacity of the previous page. /// /// Each returned `Ref` holds a reference count to this `Arc`. pages: [Arc<Page<T>>; NUM_PAGES], } /// References a slot in the slab. Indexing a slot using an `Address` is memory /// safe even if the slot has been released or the page has been deallocated. /// However, it is not guaranteed that the slot has not been reused and is now /// represents a different value. /// /// The I/O driver uses a counter to track the slot's generation. Once accessing /// the slot, the generations are compared. If they match, the value matches the /// address. #[derive(Debug, Copy, Clone, PartialEq, Eq)] pub(crate) struct Address(usize); /// An entry in the slab. pub(crate) trait Entry: Default { /// Reset the entry's value and track the generation. fn reset(&self); } /// A reference to a value stored in the slab pub(crate) struct Ref<T> { value: *const Value<T>, } /// Maximum number of pages a slab can contain. const NUM_PAGES: usize = 19; /// Minimum number of slots a page can contain. const PAGE_INITIAL_SIZE: usize = 32; const PAGE_INDEX_SHIFT: u32 = PAGE_INITIAL_SIZE.trailing_zeros() + 1; /// A page in the slab struct Page<T> { /// Slots slots: Mutex<Slots<T>>, // Number of slots currently being used. This is not guaranteed to be up to // date and should only be used as a hint. used: AtomicUsize, // Set to `true` when the page has been allocated. allocated: AtomicBool, // The number of slots the page can hold. len: usize, // Length of all previous pages combined prev_len: usize, } struct CachedPage<T> { /// Pointer to the page's slots. slots: *const Slot<T>, /// Number of initialized slots. init: usize, } /// Page state struct Slots<T> { /// Slots slots: Vec<Slot<T>>, head: usize, /// Number of slots currently in use. used: usize, } unsafe impl<T: Sync> Sync for Page<T> {} unsafe impl<T: Sync> Send for Page<T> {} unsafe impl<T: Sync> Sync for CachedPage<T> {} unsafe impl<T: Sync> Send for CachedPage<T> {} unsafe impl<T: Sync> Sync for Ref<T> {} unsafe impl<T: Sync> Send for Ref<T> {} /// A slot in the slab. Contains slot-specific metadata. /// /// `#[repr(C)]` guarantees that the struct starts w/ `value`. We use pointer /// math to map a value pointer to an index in the page. #[repr(C)] struct Slot<T> { /// Pointed to by `Ref`. value: UnsafeCell<Value<T>>, /// Next entry in the free list. next: u32, } /// Value paired with a reference to the page struct Value<T> { /// Value stored in the value value: T, /// Pointer to the page containing the slot. /// /// A raw pointer is used as this creates a ref cycle. page: *const Page<T>, } impl<T> Slab<T> { /// Create a new, empty, slab pub(crate) fn new() -> Slab<T> { // Initializing arrays is a bit annoying. Instead of manually writing // out an array and every single entry, `Default::default()` is used to // initialize the array, then the array is iterated and each value is // initialized. let mut slab = Slab { pages: Default::default(), cached: Default::default(), }; let mut len = PAGE_INITIAL_SIZE; let mut prev_len: usize = 0; for page in &mut slab.pages { let page = Arc::get_mut(page).unwrap(); page.len = len; page.prev_len = prev_len; len *= 2; prev_len += page.len; // Ensure we don't exceed the max address space. debug_assert!( page.len - 1 + page.prev_len < (1 << 24), "max = {:b}", page.len - 1 + page.prev_len ); } slab } /// Returns a new `Allocator`. /// /// The `Allocator` supports concurrent allocation of objects. pub(crate) fn allocator(&self) -> Allocator<T> { Allocator { pages: self.pages.clone(), } } /// Returns a reference to the value stored at the given address. /// /// `&mut self` is used as the call may update internal cached state. pub(crate) fn get(&mut self, addr: Address) -> Option<&T> { let page_idx = addr.page(); let slot_idx = self.pages[page_idx].slot(addr); // If the address references a slot that was last seen as uninitialized, // the `CachedPage` is updated. This requires acquiring the page lock // and updating the slot pointer and initialized offset. if self.cached[page_idx].init <= slot_idx { self.cached[page_idx].refresh(&self.pages[page_idx]); } // If the address **still** references an uninitialized slot, then the // address is invalid and `None` is returned. if self.cached[page_idx].init <= slot_idx { return None; } // Get a reference to the value. The lifetime of the returned reference // is bound to `&self`. The only way to invalidate the underlying memory // is to call `compact()`. The lifetimes prevent calling `compact()` // while references to values are outstanding. // // The referenced data is never mutated. Only `&self` references are // used and the data is `Sync`. Some(self.cached[page_idx].get(slot_idx)) } /// Calls the given function with a reference to each slot in the slab. The /// slot may not be in-use. /// /// This is used by the I/O driver during the shutdown process to notify /// each pending task. pub(crate) fn for_each(&mut self, mut f: impl FnMut(&T)) { for page_idx in 0..self.pages.len() { // It is required to avoid holding the lock when calling the // provided function. The function may attempt to acquire the lock // itself. If we hold the lock here while calling `f`, a deadlock // situation is possible. // // Instead of iterating the slots directly in `page`, which would // require holding the lock, the cache is updated and the slots are // iterated from the cache. self.cached[page_idx].refresh(&self.pages[page_idx]); for slot_idx in 0..self.cached[page_idx].init { f(self.cached[page_idx].get(slot_idx)); } } } // Release memory back to the allocator. // // If pages are empty, the underlying memory is released back to the // allocator. pub(crate) fn compact(&mut self) { // Iterate each page except the very first one. The very first page is // never freed. for (idx, page) in self.pages.iter().enumerate().skip(1) { if page.used.load(Relaxed) != 0 || !page.allocated.load(Relaxed) { // If the page has slots in use or the memory has not been // allocated then it cannot be compacted. continue; } let mut slots = match page.slots.try_lock() { Some(slots) => slots, // If the lock cannot be acquired due to being held by another // thread, don't try to compact the page. _ => continue, }; if slots.used > 0 || slots.slots.capacity() == 0 { // The page is in use or it has not yet been allocated. Either // way, there is no more work to do. continue; } page.allocated.store(false, Relaxed); // Remove the slots vector from the page. This is done so that the // freeing process is done outside of the lock's critical section. let vec = mem::replace(&mut slots.slots, vec![]); slots.head = 0; // Drop the lock so we can drop the vector outside the lock below. drop(slots); debug_assert!( self.cached[idx].slots.is_null() || self.cached[idx].slots == vec.as_ptr(), "cached = {:?}; actual = {:?}", self.cached[idx].slots, vec.as_ptr(), ); // Clear cache self.cached[idx].slots = ptr::null(); self.cached[idx].init = 0; drop(vec); } } } impl<T> fmt::Debug for Slab<T> { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { debug(fmt, "Slab", &self.pages[..]) } } impl<T: Entry> Allocator<T> { /// Allocate a new entry and return a handle to the entry. /// /// Scans pages from smallest to biggest, stopping when a slot is found. /// Pages are allocated if necessary. /// /// Returns `None` if the slab is full. pub(crate) fn allocate(&self) -> Option<(Address, Ref<T>)> { // Find the first available slot. for page in &self.pages[..] { if let Some((addr, val)) = Page::allocate(page) { return Some((addr, val)); } } None } } impl<T> fmt::Debug for Allocator<T> { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { debug(fmt, "slab::Allocator", &self.pages[..]) } } impl<T> ops::Deref for Ref<T> { type Target = T; fn deref(&self) -> &T { // Safety: `&mut` is never handed out to the underlying value. The page // is not freed until all `Ref` values are dropped. unsafe { &(*self.value).value } } } impl<T> Drop for Ref<T> { fn drop(&mut self) { // Safety: `&mut` is never handed out to the underlying value. The page // is not freed until all `Ref` values are dropped. let _ = unsafe { (*self.value).release() }; } } impl<T: fmt::Debug> fmt::Debug for Ref<T> { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { (**self).fmt(fmt) } } impl<T: Entry> Page<T> { // Allocates an object, returns the ref and address. // // `self: &Arc<Page<T>>` is avoided here as this would not work with the // loom `Arc`. fn allocate(me: &Arc<Page<T>>) -> Option<(Address, Ref<T>)> { // Before acquiring the lock, use the `used` hint. if me.used.load(Relaxed) == me.len { return None; } // Allocating objects requires synchronization let mut locked = me.slots.lock(); if locked.head < locked.slots.len() { // Re-use an already initialized slot. // // Help out the borrow checker let locked = &mut *locked; // Get the index of the slot at the head of the free stack. This is // the slot that will be reused. let idx = locked.head; let slot = &locked.slots[idx]; // Update the free stack head to point to the next slot. locked.head = slot.next as usize; // Increment the number of used slots locked.used += 1; me.used.store(locked.used, Relaxed); // Reset the slot slot.value.with(|ptr| unsafe { (*ptr).value.reset() }); // Return a reference to the slot Some((me.addr(idx), slot.gen_ref(me))) } else if me.len == locked.slots.len() { // The page is full None } else { // No initialized slots are available, but the page has more // capacity. Initialize a new slot. let idx = locked.slots.len(); if idx == 0 { // The page has not yet been allocated. Allocate the storage for // all page slots. locked.slots.reserve_exact(me.len); } // Initialize a new slot locked.slots.push(Slot { value: UnsafeCell::new(Value { value: Default::default(), page: &**me as *const _, }), next: 0, }); // Increment the head to indicate the free stack is empty locked.head += 1; // Increment the number of used slots locked.used += 1; me.used.store(locked.used, Relaxed); me.allocated.store(true, Relaxed); debug_assert_eq!(locked.slots.len(), locked.head); Some((me.addr(idx), locked.slots[idx].gen_ref(me))) } } } impl<T> Page<T> { /// Returns the slot index within the current page referenced by the given /// address. fn slot(&self, addr: Address) -> usize { addr.0 - self.prev_len } /// Returns the address for the given slot fn addr(&self, slot: usize) -> Address { Address(slot + self.prev_len) } } impl<T> Default for Page<T> { fn default() -> Page<T> { Page { used: AtomicUsize::new(0), allocated: AtomicBool::new(false), slots: Mutex::new(Slots { slots: Vec::new(), head: 0, used: 0, }), len: 0, prev_len: 0, } } } impl<T> Page<T> { /// Release a slot into the page's free list fn release(&self, value: *const Value<T>) { let mut locked = self.slots.lock(); let idx = locked.index_for(value); locked.slots[idx].next = locked.head as u32; locked.head = idx; locked.used -= 1; self.used.store(locked.used, Relaxed); } } impl<T> CachedPage<T> { /// Refresh the cache fn refresh(&mut self, page: &Page<T>) { let slots = page.slots.lock(); if !slots.slots.is_empty() { self.slots = slots.slots.as_ptr(); self.init = slots.slots.len(); } } // Get a value by index fn get(&self, idx: usize) -> &T { assert!(idx < self.init); // Safety: Pages are allocated concurrently, but are only ever // **deallocated** by `Slab`. `Slab` will always have a more // conservative view on the state of the slot array. Once `CachedPage` // sees a slot pointer and initialized offset, it will remain valid // until `compact()` is called. The `compact()` function also updates // `CachedPage`. unsafe { let slot = self.slots.add(idx); let value = slot as *const Value<T>; &(*value).value } } } impl<T> Default for CachedPage<T> { fn default() -> CachedPage<T> { CachedPage { slots: ptr::null(), init: 0, } } } impl<T> Slots<T> { /// Maps a slot pointer to an offset within the current page. /// /// The pointer math removes the `usize` index from the `Ref` struct, /// shrinking the struct to a single pointer size. The contents of the /// function is safe, the resulting `usize` is bounds checked before being /// used. /// /// # Panics /// /// panics if the provided slot pointer is not contained by the page. fn index_for(&self, slot: *const Value<T>) -> usize { use std::mem; let base = &self.slots[0] as *const _ as usize; assert!(base != 0, "page is unallocated"); let slot = slot as usize; let width = mem::size_of::<Slot<T>>(); assert!(slot >= base, "unexpected pointer"); let idx = (slot - base) / width; assert!(idx < self.slots.len() as usize); idx } } impl<T: Entry> Slot<T> { /// Generates a `Ref` for the slot. This involves bumping the page's ref count. fn gen_ref(&self, page: &Arc<Page<T>>) -> Ref<T> { // The ref holds a ref on the page. The `Arc` is forgotten here and is // resurrected in `release` when the `Ref` is dropped. By avoiding to // hold on to an explicit `Arc` value, the struct size of `Ref` is // reduced. mem::forget(page.clone()); let slot = self as *const Slot<T>; let value = slot as *const Value<T>; Ref { value } } } impl<T> Value<T> { // Release the slot, returning the `Arc<Page<T>>` logically owned by the ref. fn release(&self) -> Arc<Page<T>> { // Safety: called by `Ref`, which owns an `Arc<Page<T>>` instance. let page = unsafe { Arc::from_raw(self.page) }; page.release(self as *const _); page } } impl Address { fn page(self) -> usize { // Since every page is twice as large as the previous page, and all page // sizes are powers of two, we can determine the page index that // contains a given address by shifting the address down by the smallest // page size and looking at how many twos places necessary to represent // that number, telling us what power of two page size it fits inside // of. We can determine the number of twos places by counting the number // of leading zeros (unused twos places) in the number's binary // representation, and subtracting that count from the total number of // bits in a word. let slot_shifted = (self.0 + PAGE_INITIAL_SIZE) >> PAGE_INDEX_SHIFT; (bit::pointer_width() - slot_shifted.leading_zeros()) as usize } pub(crate) const fn as_usize(self) -> usize { self.0 } pub(crate) fn from_usize(src: usize) -> Address { Address(src) } } fn debug<T>(fmt: &mut fmt::Formatter<'_>, name: &str, pages: &[Arc<Page<T>>]) -> fmt::Result { let mut capacity = 0; let mut len = 0; for page in pages { if page.allocated.load(Relaxed) { capacity += page.len; len += page.used.load(Relaxed); } } fmt.debug_struct(name) .field("len", &len) .field("capacity", &capacity) .finish() } #[cfg(all(test, not(loom)))] mod test { use super::*; use std::sync::atomic::AtomicUsize; use std::sync::atomic::Ordering::SeqCst; struct Foo { cnt: AtomicUsize, id: AtomicUsize, } impl Default for Foo { fn default() -> Foo { Foo { cnt: AtomicUsize::new(0), id: AtomicUsize::new(0), } } } impl Entry for Foo { fn reset(&self) { self.cnt.fetch_add(1, SeqCst); } } #[test] fn insert_remove() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let (addr1, foo1) = alloc.allocate().unwrap(); foo1.id.store(1, SeqCst); assert_eq!(0, foo1.cnt.load(SeqCst)); let (addr2, foo2) = alloc.allocate().unwrap(); foo2.id.store(2, SeqCst); assert_eq!(0, foo2.cnt.load(SeqCst)); assert_eq!(1, slab.get(addr1).unwrap().id.load(SeqCst)); assert_eq!(2, slab.get(addr2).unwrap().id.load(SeqCst)); drop(foo1); assert_eq!(1, slab.get(addr1).unwrap().id.load(SeqCst)); let (addr3, foo3) = alloc.allocate().unwrap(); assert_eq!(addr3, addr1); assert_eq!(1, foo3.cnt.load(SeqCst)); foo3.id.store(3, SeqCst); assert_eq!(3, slab.get(addr3).unwrap().id.load(SeqCst)); drop(foo2); drop(foo3); slab.compact(); // The first page is never released assert!(slab.get(addr1).is_some()); assert!(slab.get(addr2).is_some()); assert!(slab.get(addr3).is_some()); } #[test] fn insert_many() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let mut entries = vec![]; for i in 0..10_000 { let (addr, val) = alloc.allocate().unwrap(); val.id.store(i, SeqCst); entries.push((addr, val)); } for (i, (addr, v)) in entries.iter().enumerate() { assert_eq!(i, v.id.load(SeqCst)); assert_eq!(i, slab.get(*addr).unwrap().id.load(SeqCst)); } entries.clear(); for i in 0..10_000 { let (addr, val) = alloc.allocate().unwrap(); val.id.store(10_000 - i, SeqCst); entries.push((addr, val)); } for (i, (addr, v)) in entries.iter().enumerate() { assert_eq!(10_000 - i, v.id.load(SeqCst)); assert_eq!(10_000 - i, slab.get(*addr).unwrap().id.load(SeqCst)); } } #[test] fn insert_drop_reverse() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let mut entries = vec![]; for i in 0..10_000 { let (addr, val) = alloc.allocate().unwrap(); val.id.store(i, SeqCst); entries.push((addr, val)); } for _ in 0..10 { // Drop 1000 in reverse for _ in 0..1_000 { entries.pop(); } // Check remaining for (i, (addr, v)) in entries.iter().enumerate() { assert_eq!(i, v.id.load(SeqCst)); assert_eq!(i, slab.get(*addr).unwrap().id.load(SeqCst)); } } } #[test] fn no_compaction_if_page_still_in_use() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let mut entries1 = vec![]; let mut entries2 = vec![]; for i in 0..10_000 { let (addr, val) = alloc.allocate().unwrap(); val.id.store(i, SeqCst); if i % 2 == 0 { entries1.push((addr, val, i)); } else { entries2.push(val); } } drop(entries2); for (addr, _, i) in &entries1 { assert_eq!(*i, slab.get(*addr).unwrap().id.load(SeqCst)); } } #[test] fn compact_all() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let mut entries = vec![]; for _ in 0..2 { entries.clear(); for i in 0..10_000 { let (addr, val) = alloc.allocate().unwrap(); val.id.store(i, SeqCst); entries.push((addr, val)); } let mut addrs = vec![]; for (addr, _) in entries.drain(..) { addrs.push(addr); } slab.compact(); // The first page is never freed for addr in &addrs[PAGE_INITIAL_SIZE..] { assert!(slab.get(*addr).is_none()); } } } #[test] fn issue_3014() { let mut slab = Slab::<Foo>::new(); let alloc = slab.allocator(); let mut entries = vec![]; for _ in 0..5 { entries.clear(); // Allocate a few pages + 1 for i in 0..(32 + 64 + 128 + 1) { let (addr, val) = alloc.allocate().unwrap(); val.id.store(i, SeqCst); entries.push((addr, val, i)); } for (addr, val, i) in &entries { assert_eq!(*i, val.id.load(SeqCst)); assert_eq!(*i, slab.get(*addr).unwrap().id.load(SeqCst)); } // Release the last entry entries.pop(); // Compact slab.compact(); // Check all the addresses for (addr, val, i) in &entries { assert_eq!(*i, val.id.load(SeqCst)); assert_eq!(*i, slab.get(*addr).unwrap().id.load(SeqCst)); } } } }