[cadcdev-svn-commits] [SCM] branch master updated. 22c8c2063dc13ddfcf717aa3f5a335f8f0d493ad

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- Log -----------------------------------------------------------------
commit 22c8c2063dc13ddfcf717aa3f5a335f8f0d493ad
Author: Falco Girgis <gyr...@gm...>
Date:   Fri Oct 11 03:50:26 2024 -0500

    Fixed Thread Starvation Issue with Priority Boosting (#803)
    
    * Fixed thread starvation issue with priority boost
    - Added comments
    - Added #define for pseudo-IRQ Thread for sake of my sanity. :)
    
    * Only reschedule threads in STATE_READY. Previously it would allow sleeping/waiting threads to be rescheduled when they weren't in the runnable queue to begin with. Now *only* STATE_READY threads, which also helps by excluding zombie/finished threads.
    
    * Only reset priority back if you are releasing the last lock a thread holds. Previously recursive mutexes may have reverted priority after locally unlocking but before waking the thread that boosted them.

-----------------------------------------------------------------------

Summary of changes:
 kernel/thread/mutex.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/kernel/thread/mutex.c b/kernel/thread/mutex.c
index 3d438965..4490ad63 100644
--- a/kernel/thread/mutex.c
+++ b/kernel/thread/mutex.c
@@ -2,6 +2,7 @@
 
    mutex.c
    Copyright (C) 2012, 2015 Lawrence Sebald
+   Copyright (C) 2024 Paul Cercueil
 
 */
 
@@ -16,6 +17,9 @@
 #include <arch/irq.h>
 #include <arch/timer.h>
 
+/* Thread pseudo-ptr representing an active IRQ context. */
+#define IRQ_THREAD  ((kthread_t *)0xFFFFFFFF)
+
 mutex_t *mutex_create(void) {
     mutex_t *rv;
 
@@ -133,13 +137,17 @@ int mutex_lock_timed(mutex_t *m, int timeout) {
             deadline = timer_ms_gettime64() + timeout;
 
         for(;;) {
+            /* Check whether we should boost priority. */
             if (m->holder->prio >= thd_current->prio) {
                 m->holder->prio = thd_current->prio;
 
-                /* Thread list is sorted by priority, update the position
-                 * of the thread holding the lock */
-                thd_remove_from_runnable(m->holder);
-                thd_add_to_runnable(m->holder, true);
+                /* Reschedule if currently scheduled. */
+                if(m->holder->state == STATE_READY) {
+                    /* Thread list is sorted by priority, update the position
+                     * of the thread holding the lock */
+                    thd_remove_from_runnable(m->holder);
+                    thd_add_to_runnable(m->holder, true);
+                }
             }
 
             rv = genwait_wait(m, timeout ? "mutex_lock_timed" : "mutex_lock",
@@ -181,7 +189,7 @@ int mutex_trylock(mutex_t *m) {
     /* If we're inside of an interrupt, pick a special value for the thread that
        would otherwise be impossible... */
     if(irq_inside_int())
-        thd = (kthread_t *)0xFFFFFFFF;
+        thd = IRQ_THREAD;
 
     if(m->type < MUTEX_TYPE_NORMAL || m->type > MUTEX_TYPE_RECURSIVE) {
         errno = EINVAL;
@@ -262,12 +270,14 @@ static int mutex_unlock_common(mutex_t *m, kthread_t *thd) {
             return -1;
     }
 
-    if (thd != (kthread_t *)0xffffffff)
-        thd->prio = thd->real_prio;
-
     /* If we need to wake up a thread, do so. */
-    if(wakeup)
+    if(wakeup) {
+        /* Restore real priority in case we were dynamically boosted. */
+        if (thd != IRQ_THREAD)
+            thd->prio = thd->real_prio;
+
         genwait_wake_one(m);
+    }
 
     return 0;
 }
@@ -278,7 +288,7 @@ int mutex_unlock(mutex_t *m) {
     /* If we're inside of an interrupt, use the special value for the thread
        from mutex_trylock(). */
     if(irq_inside_int())
-        thd = (kthread_t *)0xFFFFFFFF;
+        thd = IRQ_THREAD;
 
     return mutex_unlock_common(m, thd);
 }


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