_ 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
   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.
  10 ///
  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
  14 /// a read lock.
  15 pub struct FairRwLock<T> {
  16         lock: RwLock<T>,
  17         waiting_writers: AtomicUsize,
  18 }
  19
  20 impl<T> FairRwLock<T> {
  21         pub fn new(t: T) -> Self {
  22                 Self { lock: RwLock::new(t), waiting_writers: AtomicUsize::new(0) }
  23         }
  24
  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
  27         // memory.
  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);
  32                 res
  33         }
  34
  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();
  38                 }
  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
  43                 // codebase.
  44                 self.lock.read()
  45         }
  46
  47         pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
  48                 self.lock.try_write()
  49         }
  50 }