From b38983c1b7afe9f791dc67a33627069cce245e7e Mon Sep 17 00:00:00 2001
From: plutooo <plutooo@users.noreply.github.com>
Date: Thu, 13 Dec 2018 01:01:42 +0100
Subject: [PATCH] wait: Fix a race

---
 nx/source/kernel/uevent.c |  2 +-
 nx/source/kernel/utimer.c |  2 +-
 nx/source/kernel/wait.h   | 13 ++++++++++---
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/nx/source/kernel/uevent.c b/nx/source/kernel/uevent.c
index 8c96ae8c..9ac277ed 100644
--- a/nx/source/kernel/uevent.c
+++ b/nx/source/kernel/uevent.c
@@ -39,7 +39,7 @@ void _ueventTryAutoClear(UsermodeEvent* e)
 
 bool _ueventAddListener(UsermodeEvent* e, WaiterNode* w, size_t idx, size_t* idx_out, Handle thread)
 {
-    _waiterNodeInitialize(w, WaiterNodeType_Event, &e->waitable, thread, idx, idx_out);
+    _waiterNodeCreate(w, WaiterNodeType_Event, &e->waitable, thread, idx, idx_out);
 
     mutexLock(&e->waitable.mutex);
 
diff --git a/nx/source/kernel/utimer.c b/nx/source/kernel/utimer.c
index 998ab506..eddacca5 100644
--- a/nx/source/kernel/utimer.c
+++ b/nx/source/kernel/utimer.c
@@ -73,7 +73,7 @@ u64 _utimerGetNextTick(UsermodeTimer* t)
 
 void _utimerAddListener(UsermodeTimer* t, WaiterNode* w, size_t idx, size_t* idx_out, Handle thread)
 {
-    _waiterNodeInitialize(w, WaiterNodeType_Timer, &t->waitable, thread, idx, idx_out);
+    _waiterNodeCreate(w, WaiterNodeType_Timer, &t->waitable, thread, idx, idx_out);
 
     mutexLock(&t->waitable.mutex);
     _waiterNodeAddToWaitable(w, &t->waitable);
diff --git a/nx/source/kernel/wait.h b/nx/source/kernel/wait.h
index 4a000b3c..abe2734b 100644
--- a/nx/source/kernel/wait.h
+++ b/nx/source/kernel/wait.h
@@ -20,12 +20,19 @@ static inline void _waitableSignalAllListeners(Waitable* ww)
         node = node->next;
         WaiterNode* w = (WaiterNode*) node;
 
-        *w->idx_out = w->idx;
-        svcCancelSynchronization(w->thread);
+        // Try to swap -1 => idx on the waiter thread.
+        // If another waitable signals simultaneously only one will win the race and insert its own idx.
+        size_t minus_one = -1;
+        bool sent_idx = __atomic_compare_exchange_n(
+            w->idx_out, &minus_one, w->idx, true, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
+
+        if (sent_idx) {
+            svcCancelSynchronization(w->thread);
+        }
     }
 }
 
-static inline void _waiterNodeInitialize(
+static inline void _waiterNodeCreate(
     WaiterNode* w, WaiterNodeType type, Waitable* parent, Handle thread,
     size_t idx, size_t* idx_out)
 {