From 38f89cb4bb0b5fb5390e859ade1b98484bebbb24 Mon Sep 17 00:00:00 2001
From: 4lDO2 <4lDO2@protonmail.com>
Date: Thu, 13 Apr 2023 15:14:38 +0200
Subject: [PATCH] Improved barrier implementation.
---
src/sync/barrier.rs | 51 ++++++++++++++++++++-------------------------
1 file changed, 23 insertions(+), 28 deletions(-)
diff --git a/src/sync/barrier.rs b/src/sync/barrier.rs
index b17786e24..31095e9ca 100644
--- a/src/sync/barrier.rs
+++ b/src/sync/barrier.rs
@@ -27,11 +27,10 @@ impl Barrier {
// waiting.
let original_count = self.original_count.get();
+ let mut new = self.waited_count.fetch_add(1, Ordering::Acquire) + 1;
loop {
- let new = self.waited_count.fetch_add(1, Ordering::Acquire) + 1;
-
- match Ord::cmp(&new, &original_count) {
+ let result = match Ord::cmp(&new, &original_count) {
cmp::Ordering::Less => {
// new < original_count, i.e. we were one of the threads that incremented the counter,
// but need to continue waiting for the last waiter to notify the others.
@@ -44,43 +43,39 @@ impl Barrier {
let _ = crate::sync::futex_wait(&self.waited_count, count, None);
}
- // When the required number of threads have called pthread_barrier_wait so waited_count
- // >= original_count (should never be able to exceed that value), we can safely reset
- // the counter to zero.
-
- if self.notified_count.fetch_add(1, Ordering::Relaxed) + 1 >= original_count {
- self.waited_count.store(0, Ordering::Relaxed);
- }
-
- return WaitResult::Waited;
+ WaitResult::Waited
}
cmp::Ordering::Equal => {
// new == original_count, i.e. we were the one thread doing the last increment, and we
// will be responsible for waking up all other waiters.
- crate::sync::futex_wake(&self.waited_count, i32::MAX);
-
- if self.notified_count.fetch_add(1, Ordering::Relaxed) + 1 >= original_count {
- self.waited_count.store(0, Ordering::Relaxed);
- }
+ crate::sync::futex_wake(&self.waited_count, original_count as i32 - 1);
- return WaitResult::NotifiedAll;
+ WaitResult::NotifiedAll
}
- // FIXME: Starvation?
cmp::Ordering::Greater => {
- let mut cached = new;
- while cached >= original_count {
- // new > original_count, i.e. we are waiting on a barrier that is already finished, but
- // which has not yet awoken all its waiters and re-initialized the self. The
- // simplest way to handle this is to wait for waited_count to return to zero, and
- // start over.
+ crate::sync::futex_wait(&self.waited_count, new, None);
- crate::sync::futex_wait(&self.waited_count, cached, None);
+ new = self.waited_count.load(Ordering::Acquire);
- cached = self.waited_count.load(Ordering::Acquire);
- }
+ continue;
+ }
+ };
+
+ // When the required number of threads have called pthread_barrier_wait so waited_count
+ // >= original_count (should never be able to exceed that value), we can safely reset
+ // the counter to zero.
+
+ // TODO: Orderings
+ if self.notified_count.fetch_add(1, Ordering::AcqRel) + 1 >= original_count {
+ self.notified_count.store(0, Ordering::Release);
+ let next = self.waited_count.fetch_sub(original_count, Ordering::Release) - original_count;
+
+ if next >= original_count {
+ let _ = crate::sync::futex_wake(&self.waited_count, original_count as i32);
}
}
+ break result;
}
}
}
--
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