sched: Handle priority boosted tasks proper in setscheduler()

Ronny reported that the following scenario is not handled correctly:

	T1 (prio = 10)
	   lock(rtmutex);

	T2 (prio = 20)
	   lock(rtmutex)
	      boost T1

	T1 (prio = 20)
	   sys_set_scheduler(prio = 30)
	   T1 prio = 30
	   ....
	   sys_set_scheduler(prio = 10)
	   T1 prio = 30

The last step is wrong as T1 should now be back at prio 20.

Commit c365c292d059 ("sched: Consider pi boosting in setscheduler()")
only handles the case where a boosted tasks tries to lower its
priority.

Fix it by taking the new effective priority into account for the
decision whether a change of the priority is required.

Reported-by: Ronny Meeus <ronny.meeus@gmail.com>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: <stable@vger.kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Fixes: c365c292d059 ("sched: Consider pi boosting in setscheduler()")
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1505051806060.4225@nanos
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 21 insertions, 17 deletions

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index b73279367087..b025295f4966 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -265,15 +265,17 @@ struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
 }
 
 /*
- * Called by sched_setscheduler() to check whether the priority change
- * is overruled by a possible priority boosting.
+ * Called by sched_setscheduler() to get the priority which will be
+ * effective after the change.
  */
-int rt_mutex_check_prio(struct task_struct *task, int newprio)
+int rt_mutex_get_effective_prio(struct task_struct *task, int newprio)
 {
         if (!task_has_pi_waiters(task))
-                return 0;
+                return newprio;
 
-        return task_top_pi_waiter(task)->task->prio <= newprio;
+        if (task_top_pi_waiter(task)->task->prio <= newprio)
+                return task_top_pi_waiter(task)->task->prio;
+        return newprio;
 }
 
 /*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fe22f7510bce..34db9bf892a3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3300,15 +3300,18 @@ static void __setscheduler_params(struct task_struct *p,
 
 /* Actually do priority change: must hold pi & rq lock. */
 static void __setscheduler(struct rq *rq, struct task_struct *p,
-                           const struct sched_attr *attr)
+                           const struct sched_attr *attr, bool keep_boost)
 {
         __setscheduler_params(p, attr);
 
         /*
-         * If we get here, there was no pi waiters boosting the
-         * task. It is safe to use the normal prio.
+         * Keep a potential priority boosting if called from
+         * sched_setscheduler().
          */
-        p->prio = normal_prio(p);
+        if (keep_boost)
+                p->prio = rt_mutex_get_effective_prio(p, normal_prio(p));
+        else
+                p->prio = normal_prio(p);
 
         if (dl_prio(p->prio))
                 p->sched_class = &dl_sched_class;
@@ -3408,7 +3411,7 @@ static int __sched_setscheduler(struct task_struct *p,
         int newprio = dl_policy(attr->sched_policy) ? MAX_DL_PRIO - 1 :
                       MAX_RT_PRIO - 1 - attr->sched_priority;
         int retval, oldprio, oldpolicy = -1, queued, running;
-        int policy = attr->sched_policy;
+        int new_effective_prio, policy = attr->sched_policy;
         unsigned long flags;
         const struct sched_class *prev_class;
         struct rq *rq;
@@ -3590,15 +3593,14 @@ change:
         oldprio = p->prio;
 
         /*
-         * Special case for priority boosted tasks.
-         *
-         * If the new priority is lower or equal (user space view)
-         * than the current (boosted) priority, we just store the new
+         * Take priority boosted tasks into account. If the new
+         * effective priority is unchanged, we just store the new
          * normal parameters and do not touch the scheduler class and
          * the runqueue. This will be done when the task deboost
          * itself.
          */
-        if (rt_mutex_check_prio(p, newprio)) {
+        new_effective_prio = rt_mutex_get_effective_prio(p, newprio);
+        if (new_effective_prio == oldprio) {
                 __setscheduler_params(p, attr);
                 task_rq_unlock(rq, p, &flags);
                 return 0;
@@ -3612,7 +3614,7 @@ change:
                 put_prev_task(rq, p);
 
         prev_class = p->sched_class;
-        __setscheduler(rq, p, attr);
+        __setscheduler(rq, p, attr, true);
 
         if (running)
                 p->sched_class->set_curr_task(rq);
@@ -7346,7 +7348,7 @@ static void normalize_task(struct rq *rq, struct task_struct *p)
         queued = task_on_rq_queued(p);
         if (queued)
                 dequeue_task(rq, p, 0);
-        __setscheduler(rq, p, &attr);
+        __setscheduler(rq, p, &attr, false);
         if (queued) {
                 enqueue_task(rq, p, 0);
                 resched_curr(rq);