From 9a255dd5f92185ba0459f265c73cf7834d956e40 Mon Sep 17 00:00:00 2001
From: Florian Schmaus <flow@cs.fau.de>
Date: Wed, 5 May 2021 17:47:08 +0200
Subject: [PATCH] [Fiber] Improve memory order of reference counter
---
emper/Fiber.hpp | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/emper/Fiber.hpp b/emper/Fiber.hpp
index 6763e343..8fd61a78 100644
--- a/emper/Fiber.hpp
+++ b/emper/Fiber.hpp
@@ -91,15 +91,23 @@ class ALIGN_TO_CACHE_LINE Fiber : public Logger<LogSubsystem::F> {
return runnable.exchange(false);
}
- inline auto doAtomicIncrRefCount() -> unsigned int {
+ inline void doAtomicIncrRefCount() {
assert(referenceCounter < UINT_MAX);
isMulti = true;
- return ++referenceCounter;
+ // Note: Although the first impulse is to prevent the re-ordering
+ // of the isMulti store after the reference counter modification,
+ // this *should* not be necessary. The sychronization point where
+ // this ordering is guranteed is, when the fiber is put in a
+ // concurrent queue. There the queue must gurantee that all
+ // previous stores are visible at the time the fiber appears in
+ // the queue to other threads.
+ referenceCounter.fetch_add(1, std::memory_order_relaxed);
}
inline auto doAtomicDecrRefCount() -> unsigned int {
assert(referenceCounter > 0);
- return --referenceCounter;
+ auto previous = referenceCounter.fetch_sub(1, std::memory_order_acq_rel);
+ return previous - 1;
}
template <LogSubsystem>
--
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