sched/rtmutex/deadline: Fix a PI crash for deadline tasks

A crash happened while I was playing with deadline PI rtmutex.

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
    IP: [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
    PGD 232a75067 PUD 230947067 PMD 0
    Oops: 0000 [#1] SMP
    CPU: 1 PID: 10994 Comm: a.out Not tainted

    Call Trace:
    [<ffffffff810b658c>] enqueue_task+0x2c/0x80
    [<ffffffff810ba763>] activate_task+0x23/0x30
    [<ffffffff810d0ab5>] pull_dl_task+0x1d5/0x260
    [<ffffffff810d0be6>] pre_schedule_dl+0x16/0x20
    [<ffffffff8164e783>] __schedule+0xd3/0x900
    [<ffffffff8164efd9>] schedule+0x29/0x70
    [<ffffffff8165035b>] __rt_mutex_slowlock+0x4b/0xc0
    [<ffffffff81650501>] rt_mutex_slowlock+0xd1/0x190
    [<ffffffff810eeb33>] rt_mutex_timed_lock+0x53/0x60
    [<ffffffff810ecbfc>] futex_lock_pi.isra.18+0x28c/0x390
    [<ffffffff810ed8b0>] do_futex+0x190/0x5b0
    [<ffffffff810edd50>] SyS_futex+0x80/0x180

This is because rt_mutex_enqueue_pi() and rt_mutex_dequeue_pi()
are only protected by pi_lock when operating pi waiters, while
rt_mutex_get_top_task(), will access them with rq lock held but
not holding pi_lock.

In order to tackle it, we introduce new "pi_top_task" pointer
cached in task_struct, and add new rt_mutex_update_top_task()
to update its value, it can be called by rt_mutex_setprio()
which held both owner's pi_lock and rq lock. Thus "pi_top_task"
can be safely accessed by enqueue_task_dl() under rq lock.

Originally-From: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Xunlei Pang <xlpang@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170323150216.157682758@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

3 files changed, 24 insertions, 8 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 6c463c80e93d..b30196a00b0d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1438,6 +1438,7 @@ static void rt_mutex_init_task(struct task_struct *p)
 #ifdef CONFIG_RT_MUTEXES
 	p->pi_waiters = RB_ROOT;
 	p->pi_waiters_leftmost = NULL;
+	p->pi_top_task = NULL;
 	p->pi_blocked_on = NULL;
 #endif
 }
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 71ecf0624410..bc05b104eaed 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -323,6 +323,19 @@ rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
 }
 
 /*
+ * Must hold both p->pi_lock and task_rq(p)->lock.
+ */
+void rt_mutex_update_top_task(struct task_struct *p)
+{
+	if (!task_has_pi_waiters(p)) {
+		p->pi_top_task = NULL;
+		return;
+	}
+
+	p->pi_top_task = task_top_pi_waiter(p)->task;
+}
+
+/*
  * Calculate task priority from the waiter tree priority
  *
  * Return task->normal_prio when the waiter tree is empty or when
@@ -337,12 +350,12 @@ int rt_mutex_getprio(struct task_struct *task)
 		   task->normal_prio);
 }
 
+/*
+ * Must hold either p->pi_lock or task_rq(p)->lock.
+ */
 struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
 {
-	if (likely(!task_has_pi_waiters(task)))
-		return NULL;
-
-	return task_top_pi_waiter(task)->task;
+	return task->pi_top_task;
 }
 
 /*
@@ -351,12 +364,12 @@ struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
  */
 int rt_mutex_get_effective_prio(struct task_struct *task, int newprio)
 {
-	if (!task_has_pi_waiters(task))
+	struct task_struct *top_task = rt_mutex_get_top_task(task);
+
+	if (!top_task)
 		return newprio;
 
-	if (task_top_pi_waiter(task)->task->prio <= newprio)
-		return task_top_pi_waiter(task)->task->prio;
-	return newprio;
+	return min(top_task->prio, newprio);
 }
 
 /*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ab9f6ac099a7..e1f44ec701c8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3713,6 +3713,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 		goto out_unlock;
 	}
 
+	rt_mutex_update_top_task(p);
+
 	trace_sched_pi_setprio(p, prio);
 	oldprio = p->prio;