From 202bcf5b101dd8497f147e1163e6ad7a262fb600 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Thu, 23 Apr 2026 06:07:51 -0500
Subject: [PATCH] [scheduler] Use __atomic_load_n only on reader fast-path
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Walk back the __atomic_store_n on the writer paths — the mutators
already hold lock_, so plain counter_++/=/+=/-- is sufficient to
serialise against other writers. The reader fast-path still uses
__atomic_load_n(&counter, __ATOMIC_RELAXED) to express concurrent-
read intent in the C++ memory model and keep the compiler from
caching/eliding the read. On ARMv5TE it compiles to a plain LDR —
same codegen as before.
---
 esphome/core/scheduler.h | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/esphome/core/scheduler.h b/esphome/core/scheduler.h
index a6ee079c3a..d5d1a0385c 100644
--- a/esphome/core/scheduler.h
+++ b/esphome/core/scheduler.h
@@ -550,16 +550,15 @@ class Scheduler {
   }
 
   // Increment to_add_count_ (no-op on single-threaded platforms).
-  // On NO_ATOMICS the caller must hold lock_ to serialise the load-modify-store
-  // against other writers; the __atomic_store_n makes the write visible to
-  // concurrent readers in the C++ memory model.
+  // On NO_ATOMICS the caller must hold lock_ to serialise RMW against
+  // other writers; reader fast-path uses __atomic_load_n.
   void to_add_count_increment_locked_() {
 #ifdef ESPHOME_THREAD_SINGLE
     // No counter needed — to_add_empty_() checks the vector directly
 #elif defined(ESPHOME_THREAD_MULTI_ATOMICS)
     this->to_add_count_.fetch_add(1, std::memory_order_relaxed);
 #else
-  __atomic_store_n(&this->to_add_count_, __atomic_load_n(&this->to_add_count_, __ATOMIC_RELAXED) + 1, __ATOMIC_RELAXED);
+  this->to_add_count_++;
 #endif
   }
 
@@ -570,7 +569,7 @@ class Scheduler {
 #elif defined(ESPHOME_THREAD_MULTI_ATOMICS)
     this->to_add_count_.store(0, std::memory_order_relaxed);
 #else
-  __atomic_store_n(&this->to_add_count_, 0, __ATOMIC_RELAXED);
+  this->to_add_count_ = 0;
 #endif
   }
 
@@ -602,7 +601,7 @@ class Scheduler {
 #ifdef ESPHOME_THREAD_MULTI_ATOMICS
     this->defer_count_.fetch_add(1, std::memory_order_relaxed);
 #else
-    __atomic_store_n(&this->defer_count_, __atomic_load_n(&this->defer_count_, __ATOMIC_RELAXED) + 1, __ATOMIC_RELAXED);
+    this->defer_count_++;
 #endif
   }
 
@@ -610,7 +609,7 @@ class Scheduler {
 #ifdef ESPHOME_THREAD_MULTI_ATOMICS
     this->defer_count_.store(0, std::memory_order_relaxed);
 #else
-    __atomic_store_n(&this->defer_count_, 0, __ATOMIC_RELAXED);
+    this->defer_count_ = 0;
 #endif
   }
 
@@ -639,30 +638,24 @@ class Scheduler {
   void to_remove_add_locked_(uint32_t count) {
 #ifdef ESPHOME_THREAD_MULTI_ATOMICS
     this->to_remove_.fetch_add(count, std::memory_order_relaxed);
-#elif defined(ESPHOME_THREAD_MULTI_NO_ATOMICS)
-    __atomic_store_n(&this->to_remove_, __atomic_load_n(&this->to_remove_, __ATOMIC_RELAXED) + count, __ATOMIC_RELAXED);
 #else
-  this->to_remove_ += count;
+    this->to_remove_ += count;
 #endif
   }
 
   void to_remove_decrement_locked_() {
 #ifdef ESPHOME_THREAD_MULTI_ATOMICS
     this->to_remove_.fetch_sub(1, std::memory_order_relaxed);
-#elif defined(ESPHOME_THREAD_MULTI_NO_ATOMICS)
-    __atomic_store_n(&this->to_remove_, __atomic_load_n(&this->to_remove_, __ATOMIC_RELAXED) - 1, __ATOMIC_RELAXED);
 #else
-  this->to_remove_--;
+    this->to_remove_--;
 #endif
   }
 
   void to_remove_clear_locked_() {
 #ifdef ESPHOME_THREAD_MULTI_ATOMICS
     this->to_remove_.store(0, std::memory_order_relaxed);
-#elif defined(ESPHOME_THREAD_MULTI_NO_ATOMICS)
-    __atomic_store_n(&this->to_remove_, 0, __ATOMIC_RELAXED);
 #else
-  this->to_remove_ = 0;
+    this->to_remove_ = 0;
 #endif
   }