_ Git - rust-lightning/blob - lightning/src/sync/fairrwlock.rs

   1 use std::sync::{LockResult, RwLock, RwLockReadGuard, RwLockWriteGuard, TryLockResult};
   2 use std::sync::atomic::{AtomicUsize, Ordering};
   3 use super::{LockHeldState, LockTestExt};
   4
   5 /// Rust libstd's RwLock does not provide any fairness guarantees (and, in fact, when used on
   6 /// Linux with pthreads under the hood, readers trivially and completely starve writers).
   7 /// Because we often hold read locks while doing message processing in multiple threads which
   8 /// can use significant CPU time, with write locks being time-sensitive but relatively small in
   9 /// CPU time, we can end up with starvation completely blocking incoming connections or pings,
  10 /// especially during initial graph sync.
  11 ///
  12 /// Thus, we need to block readers when a writer is pending, which we do with a trivial RwLock
  13 /// wrapper here. Its not particularly optimized, but provides some reasonable fairness by
  14 /// blocking readers (by taking the write lock) if there are writers pending when we go to take
  15 /// a read lock.
  16 pub struct FairRwLock<T> {
  17         lock: RwLock<T>,
  18         waiting_writers: AtomicUsize,
  19 }
  20
  21 impl<T> FairRwLock<T> {
  22         pub fn new(t: T) -> Self {
  23                 Self { lock: RwLock::new(t), waiting_writers: AtomicUsize::new(0) }
  24         }
  25
  26         // Note that all atomic accesses are relaxed, as we do not rely on the atomics here for any
  27         // ordering at all, instead relying on the underlying RwLock to provide ordering of unrelated
  28         // memory.
  29         pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
  30                 self.waiting_writers.fetch_add(1, Ordering::Relaxed);
  31                 let res = self.lock.write();
  32                 self.waiting_writers.fetch_sub(1, Ordering::Relaxed);
  33                 res
  34         }
  35
  36         pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
  37                 if self.waiting_writers.load(Ordering::Relaxed) != 0 {
  38                         let _write_queue_lock = self.lock.write();
  39                 }
  40                 // Note that we don't consider ensuring that an underlying RwLock allowing writers to
  41                 // starve readers doesn't exhibit the same behavior here. I'm not aware of any
  42                 // libstd-backing RwLock which exhibits this behavior, and as documented in the
  43                 // struct-level documentation, it shouldn't pose a significant issue for our current
  44                 // codebase.
  45                 self.lock.read()
  46         }
  47
  48         pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
  49                 self.lock.try_write()
  50         }
  51 }
  52
  53 impl<'a, T: 'a> LockTestExt<'a> for FairRwLock<T> {
  54         #[inline]
  55         fn held_by_thread(&self) -> LockHeldState {
  56                 // fairrwlock is only built in non-test modes, so we should never support tests.
  57                 LockHeldState::Unsupported
  58         }
  59         type ExclLock = RwLockWriteGuard<'a, T>;
  60         #[inline]
  61         fn unsafe_well_ordered_double_lock_self(&'a self) -> RwLockWriteGuard<'a, T> {
  62                 self.write().unwrap()
  63         }
  64 }