1. When trying to acquire an rtmutex, we first try to grab it without
queueing the waiter, and explicitly check for that initial attempt
in the !waiter path of __try_to_take_rt_mutex(). Checking whether
the lock taker is top waiter before allowing a steal attempt in that
path is a thinko: the lock taker has not yet blocked.
2. It seems wrong to change the definition of rt_mutex_waiter_less()
to mean less or perhaps equal when we have an rt_mutex_waiter_equal().
Remove the thinko, and implement rt_mutex_stealable(steal_mode) using
a restored rt_mutex_waiter_less(), and rt_mutex_waiter_equal().
Signed-off-by: Mike Galbraith <efault@xxxxxx>
Fixes: f34d077c6f28 ("rt: Add the preempt-rt lock replacement APIs")
---
kernel/locking/rtmutex.c | 69 +++++++++++++++++++++++------------------------
1 file changed, 35 insertions(+), 34 deletions(-)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -236,26 +236,19 @@ static inline bool unlock_rt_mutex_safe(
}
#endif
-#define STEAL_NORMAL 0
-#define STEAL_LATERAL 1
-
/*
* Only use with rt_mutex_waiter_{less,equal}()
*/
-#define task_to_waiter(p) \
- &(struct rt_mutex_waiter){ .prio = (p)->prio, .deadline = (p)->dl.deadline }
+#define task_to_waiter(p) &(struct rt_mutex_waiter) \
+ { .prio = (p)->prio, .deadline = (p)->dl.deadline, .task = (p) }
static inline int
rt_mutex_waiter_less(struct rt_mutex_waiter *left,
- struct rt_mutex_waiter *right, int mode)
+ struct rt_mutex_waiter *right)
{
- if (mode == STEAL_NORMAL) {
- if (left->prio < right->prio)
- return 1;
- } else {
- if (left->prio <= right->prio)
- return 1;
- }
+ if (left->prio < right->prio)
+ return 1;
+
/*
* If both waiters have dl_prio(), we check the deadlines of the
* associated tasks.
@@ -287,6 +280,25 @@ rt_mutex_waiter_equal(struct rt_mutex_wa
return 1;
}
+#define STEAL_NORMAL 0
+#define STEAL_LATERAL 1
+
+static inline int rt_mutex_stealable(struct rt_mutex_waiter *thief,
+ struct rt_mutex_waiter *victim, int mode)
+{
+ if (rt_mutex_waiter_less(thief, victim))
+ return 1;
+
+ /*
+ * Note that RT tasks are excluded from lateral-steals
+ * to prevent the introduction of an unbounded latency.
+ */
+ if (mode == STEAL_NORMAL || rt_task(thief->task))
+ return 0;
+
+ return rt_mutex_waiter_equal(thief, victim);
+}
+
static void
rt_mutex_enqueue(struct rt_mutex *lock, struct rt_mutex_waiter *waiter)
{
@@ -298,7 +310,7 @@ rt_mutex_enqueue(struct rt_mutex *lock,
while (*link) {
parent = *link;
entry = rb_entry(parent, struct rt_mutex_waiter, tree_entry);
- if (rt_mutex_waiter_less(waiter, entry, STEAL_NORMAL)) {
+ if (rt_mutex_waiter_less(waiter, entry)) {
link = &parent->rb_left;
} else {
link = &parent->rb_right;
@@ -337,7 +349,7 @@ rt_mutex_enqueue_pi(struct task_struct *
while (*link) {
parent = *link;
entry = rb_entry(parent, struct rt_mutex_waiter, pi_tree_entry);
- if (rt_mutex_waiter_less(waiter, entry, STEAL_NORMAL)) {
+ if (rt_mutex_waiter_less(waiter, entry)) {
link = &parent->rb_left;
} else {
link = &parent->rb_right;
@@ -847,6 +859,7 @@ static int rt_mutex_adjust_prio_chain(st
* @task: The task which wants to acquire the lock
* @waiter: The waiter that is queued to the lock's wait tree if the
* callsite called task_blocked_on_lock(), otherwise NULL
+ * @mode: Lock steal mode (STEAL_NORMAL, STEAL_LATERAL)
*/
static int __try_to_take_rt_mutex(struct rt_mutex *lock,
struct task_struct *task,
@@ -890,10 +903,9 @@ static int __try_to_take_rt_mutex(struct
* @lock, give up.
*/
if (waiter != rt_mutex_top_waiter(lock)) {
- /* XXX rt_mutex_waiter_less() ? */
+ /* XXX rt_mutex_stealable() ? */
return 0;
}
-
/*
* We can acquire the lock. Remove the waiter from the
* lock waiters tree.
@@ -910,25 +922,14 @@ static int __try_to_take_rt_mutex(struct
* not need to be dequeued.
*/
if (rt_mutex_has_waiters(lock)) {
- struct task_struct *pown = rt_mutex_top_waiter(lock)->task;
-
- if (task != pown)
- return 0;
-
- /*
- * Note that RT tasks are excluded from lateral-steals
- * to prevent the introduction of an unbounded latency.
- */
- if (rt_task(task))
- mode = STEAL_NORMAL;
/*
- * If @task->prio is greater than or equal to
- * the top waiter priority (kernel view),
- * @task lost.
+ * If @task->prio is greater than the top waiter
+ * priority (kernel view), or equal to it when
+ * lateral steal is forbidden, @task lost.
*/
- if (!rt_mutex_waiter_less(task_to_waiter(task),
- rt_mutex_top_waiter(lock),
- mode))
+ if (!rt_mutex_stealable(task_to_waiter(task),
+ rt_mutex_top_waiter(lock),
+ mode))
return 0;
/*
* The current top waiter stays enqueued. We
--
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