1 use std::sync::{LockResult, RwLock, RwLockReadGuard, RwLockWriteGuard, TryLockResult};
2 use std::sync::atomic::{AtomicUsize, Ordering};
4 /// Rust libstd's RwLock does not provide any fairness guarantees (and, in fact, when used on
5 /// Linux with pthreads under the hood, readers trivially and completely starve writers).
6 /// Because we often hold read locks while doing message processing in multiple threads which
7 /// can use significant CPU time, with write locks being time-sensitive but relatively small in
8 /// CPU time, we can end up with starvation completely blocking incoming connections or pings,
9 /// especially during initial graph sync.
11 /// Thus, we need to block readers when a writer is pending, which we do with a trivial RwLock
12 /// wrapper here. Its not particularly optimized, but provides some reasonable fairness by
13 /// blocking readers (by taking the write lock) if there are writers pending when we go to take
15 pub struct FairRwLock<T> {
17 waiting_writers: AtomicUsize,
20 impl<T> FairRwLock<T> {
21 pub fn new(t: T) -> Self {
22 Self { lock: RwLock::new(t), waiting_writers: AtomicUsize::new(0) }
25 // Note that all atomic accesses are relaxed, as we do not rely on the atomics here for any
26 // ordering at all, instead relying on the underlying RwLock to provide ordering of unrelated
28 pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
29 self.waiting_writers.fetch_add(1, Ordering::Relaxed);
30 let res = self.lock.write();
31 self.waiting_writers.fetch_sub(1, Ordering::Relaxed);
35 pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
36 if self.waiting_writers.load(Ordering::Relaxed) != 0 {
37 let _write_queue_lock = self.lock.write();
39 // Note that we don't consider ensuring that an underlying RwLock allowing writers to
40 // starve readers doesn't exhibit the same behavior here. I'm not aware of any
41 // libstd-backing RwLock which exhibits this behavior, and as documented in the
42 // struct-level documentation, it shouldn't pose a significant issue for our current
47 pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {