232 lines
8.8 KiB
Rust
232 lines
8.8 KiB
Rust
//! The half-lock structure
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//!
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//! We need a way to protect the structure with configured hooks ‒ a signal may happen in arbitrary
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//! thread and needs to read them while another thread might be manipulating the structure.
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//!
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//! Under ordinary circumstances we would be happy to just use `Mutex<HashMap<c_int, _>>`. However,
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//! as we use it in the signal handler, we are severely limited in what we can or can't use. So we
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//! choose to implement kind of spin-look thing with atomics.
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//!
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//! In the reader it is always simply locked and then unlocked, making sure it doesn't disappear
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//! while in use.
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//!
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//! The writer has a separate mutex (that prevents other writers; this is used outside of the
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//! signal handler), makes a copy of the data and swaps an atomic pointer to the data structure.
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//! But it waits until everything is unlocked (no signal handler has the old data) for dropping the
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//! old instance. There's a generation trick to make sure that new signal locks another instance.
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//!
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//! The downside is, this is an active spin lock at the writer end. However, we assume than:
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//!
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//! * Signals are one time setup before we actually have threads. We just need to make *sure* we
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//! are safe even if this is not true.
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//! * Signals are rare, happening at the same time as the write even rarer.
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//! * Signals are short, as there is mostly nothing allowed inside them anyway.
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//! * Our tool box is severely limited.
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//!
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//! Therefore this is hopefully reasonable trade-off.
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//!
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//! # Atomic orderings
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//!
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//! The whole code uses SeqCst conservatively. Atomics are not used because of performance here and
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//! are the minor price around signals anyway. But the comments state which orderings should be
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//! enough in practice in case someone wants to get inspired (but do make your own check through
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//! them anyway).
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use std::isize;
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use std::marker::PhantomData;
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use std::ops::Deref;
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use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
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use std::sync::{Mutex, MutexGuard, PoisonError};
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use std::thread;
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use libc;
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const YIELD_EVERY: usize = 16;
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const MAX_GUARDS: usize = (isize::MAX) as usize;
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pub(crate) struct ReadGuard<'a, T: 'a> {
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data: &'a T,
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lock: &'a AtomicUsize,
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}
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impl<'a, T> Deref for ReadGuard<'a, T> {
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type Target = T;
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fn deref(&self) -> &T {
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self.data
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}
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}
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impl<'a, T> Drop for ReadGuard<'a, T> {
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fn drop(&mut self) {
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// We effectively unlock; Release would be enough.
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self.lock.fetch_sub(1, Ordering::SeqCst);
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}
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}
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pub(crate) struct WriteGuard<'a, T: 'a> {
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_guard: MutexGuard<'a, ()>,
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lock: &'a HalfLock<T>,
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data: &'a T,
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}
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impl<'a, T> WriteGuard<'a, T> {
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pub(crate) fn store(&mut self, val: T) {
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// Move to the heap and convert to raw pointer for AtomicPtr.
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let new = Box::into_raw(Box::new(val));
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self.data = unsafe { &*new };
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// We can just put the new value in here safely, we worry only about dropping the old one.
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// Release might (?) be enough, to "upload" the data.
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let old = self.lock.data.swap(new, Ordering::SeqCst);
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// Now we make sure there's no reader having the old data.
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self.lock.write_barrier();
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drop(unsafe { Box::from_raw(old) });
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}
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}
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impl<'a, T> Deref for WriteGuard<'a, T> {
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type Target = T;
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fn deref(&self) -> &T {
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// Protected by that mutex
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self.data
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}
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}
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pub(crate) struct HalfLock<T> {
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// We conceptually contain an instance of T
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_t: PhantomData<T>,
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// The actual data as a pointer.
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data: AtomicPtr<T>,
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// The generation of the data. Influences which slot of the lock counter we use.
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generation: AtomicUsize,
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// How many active locks are there?
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lock: [AtomicUsize; 2],
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// Mutex for the writers; only one writer.
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write_mutex: Mutex<()>,
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}
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impl<T> HalfLock<T> {
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pub(crate) fn new(data: T) -> Self {
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// Move to the heap so we can safely point there. Then convert to raw pointer as AtomicPtr
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// operates on raw pointers. The AtomicPtr effectively acts like Box for us semantically.
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let ptr = Box::into_raw(Box::new(data));
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Self {
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_t: PhantomData,
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data: AtomicPtr::new(ptr),
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generation: AtomicUsize::new(0),
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lock: [AtomicUsize::new(0), AtomicUsize::new(0)],
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write_mutex: Mutex::new(()),
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}
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}
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pub(crate) fn read(&self) -> ReadGuard<T> {
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// Relaxed should be enough; we only pick one or the other slot and the writer observes
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// that both were 0 at some time. So the actual value doesn't really matter for safety,
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// only the changing improves the performance.
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let gen = self.generation.load(Ordering::SeqCst);
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let lock = &self.lock[gen % 2];
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// Effectively locking something, acquire should be enough.
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let guard_cnt = lock.fetch_add(1, Ordering::SeqCst);
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// This is to prevent overflowing the counter in some degenerate cases, which could lead to
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// UB (freeing data while still in use). However, as this data structure is used only
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// internally and it's not possible to leak the guard and the guard itself takes some
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// memory, it should be really impossible to trigger this case. Still, we include it from
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// abundance of caution.
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//
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// This technically is not fully correct as enough threads being in between here and the
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// abort below could still overflow it and it could get freed for some *other* thread, but
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// that would mean having too many active threads to fit into RAM too and is even more
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// absurd corner case than the above.
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if guard_cnt > MAX_GUARDS {
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unsafe { libc::abort() };
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}
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// Acquire should be enough; we need to "download" the data, paired with the swap on the
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// same pointer.
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let data = self.data.load(Ordering::SeqCst);
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// Safe:
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// * It did point to valid data when put in.
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// * Protected by lock, so still valid.
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let data = unsafe { &*data };
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ReadGuard { data, lock }
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}
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fn update_seen(&self, seen_zero: &mut [bool; 2]) {
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for (seen, slot) in seen_zero.iter_mut().zip(&self.lock) {
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*seen = *seen || slot.load(Ordering::SeqCst) == 0;
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}
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}
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fn write_barrier(&self) {
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// Do a first check of seeing zeroes before we switch the generation. At least one of them
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// should be zero by now, due to having drained the generation before leaving the previous
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// writer.
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let mut seen_zero = [false; 2];
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self.update_seen(&mut seen_zero);
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// By switching the generation to the other slot, we make sure the currently active starts
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// draining while the other will start filling up.
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self.generation.fetch_add(1, Ordering::SeqCst); // Overflow is fine.
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let mut iter = 0usize;
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while !seen_zero.iter().all(|s| *s) {
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iter = iter.wrapping_add(1);
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// Be somewhat less aggressive while looping, switch to the other threads if possible.
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if cfg!(not(miri)) {
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if iter % YIELD_EVERY == 0 {
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thread::yield_now();
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} else {
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// Replaced by hint::spin_loop, but we want to support older compiler
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#[allow(deprecated)]
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atomic::spin_loop_hint();
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}
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}
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self.update_seen(&mut seen_zero);
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}
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}
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pub(crate) fn write(&self) -> WriteGuard<T> {
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// While it's possible the user code panics, our code in store doesn't and the data gets
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// swapped atomically. So if it panics, nothing gets changed, therefore poisons are of no
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// interest here.
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let guard = self
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.write_mutex
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.lock()
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.unwrap_or_else(PoisonError::into_inner);
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// Relaxed should be enough, as we are under the same mutex that was used to get the data
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// in.
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let data = self.data.load(Ordering::SeqCst);
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// Safe:
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// * Stored as valid data
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// * Only this method, protected by mutex, can change the pointer, so it didn't go away.
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let data = unsafe { &*data };
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WriteGuard {
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data,
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_guard: guard,
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lock: self,
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}
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}
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}
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impl<T> Drop for HalfLock<T> {
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fn drop(&mut self) {
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// During drop we are sure there are no other borrows of the data so we are free to just
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// drop it. Also, the drop impl won't be called in practice in our case, as it is used
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// solely as a global variable, but we provide it for completeness and tests anyway.
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//
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// unsafe: the pointer in there is always valid, we just take the last instance out.
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unsafe {
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// Acquire should be enough.
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let data = Box::from_raw(self.data.load(Ordering::SeqCst));
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drop(data);
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}
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}
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}
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