[PATCH -v2 9/9] rtmutex: Fix more prio comparisons

[Peter Zijlstra]

There was a pure ->prio comparison left in try_to_wake_rt_mutex(),
convert it to use rt_mutex_waiter_less(), noting that greater-or-equal
is not-less (both in kernel priority view).

This necessitated the introduction of cmp_task() which creates a
pointer to an unnamed stack variable of struct rt_mutex_waiter type to
compare against tasks.

With this, we can now also create and employ rt_mutex_waiter_equal().

Reviewed-and-tested-by: Juri Lelli <juri.lelli@xxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/locking/rtmutex.c |   32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -158,6 +158,12 @@ static inline bool unlock_rt_mutex_safe(
 }
 #endif
 
+/*
+ * Only use with rt_mutex_waiter_{less,equal}()
+ */
+#define cmp_task(p)	\
+	&(struct rt_mutex_waiter){ .prio = (p)->prio, .deadline = (p)->dl.deadline }
+
 static inline int
 rt_mutex_waiter_less(struct rt_mutex_waiter *left,
 		     struct rt_mutex_waiter *right)
@@ -177,6 +183,25 @@ rt_mutex_waiter_less(struct rt_mutex_wai
 	return 0;
 }
 
+static inline int
+rt_mutex_waiter_equal(struct rt_mutex_waiter *left,
+		      struct rt_mutex_waiter *right)
+{
+	if (left->prio != right->prio)
+		return 0;
+
+	/*
+	 * If both waiters have dl_prio(), we check the deadlines of the
+	 * associated tasks.
+	 * If left waiter has a dl_prio(), and we didn't return 0 above,
+	 * then right waiter has a dl_prio() too.
+	 */
+	if (dl_prio(left->prio))
+		return left->deadline == right->deadline;
+
+	return 1;
+}
+
 static void
 rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
 {
@@ -487,7 +512,7 @@ static int rt_mutex_adjust_prio_chain(st
 	 * enabled we continue, but stop the requeueing in the chain
 	 * walk.
 	 */
-	if (waiter->prio == task->prio && !dl_task(task)) {
+	if (rt_mutex_waiter_equal(waiter, cmp_task(task))) {
 		if (!detect_deadlock)
 			goto out_unlock_pi;
 		else
@@ -790,7 +815,8 @@ static int try_to_take_rt_mutex(struct r
 			 * the top waiter priority (kernel view),
 			 * @task lost.
 			 */
-			if (task->prio >= rt_mutex_top_waiter(lock)->prio)
+			if (!rt_mutex_waiter_less(cmp_task(task),
+						  rt_mutex_top_waiter(lock)))
 				return 0;
 
 			/*
@@ -1055,7 +1081,7 @@ void rt_mutex_adjust_pi(struct task_stru
 	raw_spin_lock_irqsave(&task->pi_lock, flags);
 
 	waiter = task->pi_blocked_on;
-	if (!waiter || (waiter->prio == task->prio && !dl_prio(task->prio))) {
+	if (!waiter || rt_mutex_waiter_equal(waiter, cmp_task(task))) {
 		raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 		return;
 	}