From b9957f0957ffc36edd194730695d20d6aa658bbc Mon Sep 17 00:00:00 2001
From: Chris Copeland <chris@chrisnc.net>
Date: Sun, 31 Mar 2024 22:42:54 -0700
Subject: [PATCH] remove rt_mutex_holder, avoid priority donation to an
 interrupt

---
 include/rt/mutex.h |  7 -------
 src/rt.c           | 15 ++++++++++-----
 src/rwlock.c       |  6 +++++-
 3 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/include/rt/mutex.h b/include/rt/mutex.h
index 8d1e9e5..b233c1e 100644
--- a/include/rt/mutex.h
+++ b/include/rt/mutex.h
@@ -50,13 +50,6 @@ struct rt_task;
 #define RT_MUTEX_RECURSIVE(name)                                               \
     struct rt_mutex name = RT_MUTEX_INIT_RECURSIVE(name)
 
-static inline struct rt_task *rt_mutex_holder(const struct rt_mutex *mutex)
-{
-    return (struct rt_task *)(rt_atomic_load(&mutex->holder,
-                                             RT_ATOMIC_RELAXED) &
-                              RT_MUTEX_HOLDER_MASK);
-}
-
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/rt.c b/src/rt.c
index 768dc1c..a26930c 100644
--- a/src/rt.c
+++ b/src/rt.c
@@ -357,23 +357,28 @@ static void mutex_donate(struct rt_mutex *mutex)
 {
     do
     {
-        struct rt_task *holder = rt_mutex_holder(mutex);
-        if (holder == NULL)
+        uintptr_t holder = rt_atomic_load(&mutex->holder, RT_ATOMIC_RELAXED);
+        if ((holder == RT_MUTEX_UNLOCKED) ||
+            (holder == RT_MUTEX_HOLDER_INTERRUPT))
         {
-            // If the mutex is not held then no donation is needed.
+            /* If the mutex is not held or held by an interrupt, then no
+             * donation is needed. */
             return;
         }
 
+        struct rt_task *const task =
+            (struct rt_task *)(holder & RT_MUTEX_HOLDER_MASK);
+
         if (!rt_list_is_empty(&mutex->wait_list))
         {
             // Re-sort the mutex in the holder's mutex list.
             rt_list_remove(&mutex->list);
-            insert_mutex_by_priority(&holder->mutex_list, mutex);
+            insert_mutex_by_priority(&task->mutex_list, mutex);
         }
 
         /* Update the holder's priority and get the next mutex to calculate
          * donation on, if any. */
-        mutex = task_donate(holder);
+        mutex = task_donate(task);
     } while (mutex != NULL);
 }
 
diff --git a/src/rwlock.c b/src/rwlock.c
index c3f20ab..330fbed 100644
--- a/src/rwlock.c
+++ b/src/rwlock.c
@@ -215,7 +215,11 @@ void rt_rwlock_wrunlock(struct rt_rwlock *lock)
 
 void rt_rwlock_unlock(struct rt_rwlock *lock)
 {
-    if (rt_mutex_holder(&lock->mutex) == rt_task_self())
+    const uintptr_t holder =
+        rt_atomic_load(&lock->mutex.holder, RT_ATOMIC_RELAXED) &
+        RT_MUTEX_HOLDER_MASK;
+    if ((holder == (uintptr_t)rt_task_self()) ||
+        (holder == RT_MUTEX_HOLDER_INTERRUPT))
     {
         rt_rwlock_wrunlock(lock);
     }