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>

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 91d9049f003938a77002d76f8a2f0b3313437294..2c487e0879d58d13f76265e9e0606b528d352425 100644 (file)
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -181,6 +181,7 @@ extern struct cred init_cred;
 #ifdef CONFIG_RT_MUTEXES
 # define INIT_RT_MUTEXES(tsk)                                          \
        .pi_waiters = RB_ROOT,                                          \
+       .pi_top_task = NULL,                                            \
        .pi_waiters_leftmost = NULL,
 #else
 # define INIT_RT_MUTEXES(tsk)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d67eee84fd430f3c44b77d4ba007ec5d2dcabb2b..1ea2eee7bc4f80e256384a0ac4d912df67e224ee 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -775,6 +775,8 @@ struct task_struct {
        /* PI waiters blocked on a rt_mutex held by this task: */
        struct rb_root                  pi_waiters;
        struct rb_node                  *pi_waiters_leftmost;
+       /* Updated under owner's pi_lock and rq lock */
+       struct task_struct              *pi_top_task;
        /* Deadlock detection and priority inheritance handling: */
        struct rt_mutex_waiter          *pi_blocked_on;
 #endif

diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h
index 3bd668414f61a593e9f40ff8364a3dce63d78d01..10ee7eeb0ee2915e11d0f3411ca0f981bba4b78a 100644 (file)
--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -21,6 +21,7 @@ static inline int rt_task(struct task_struct *p)
 extern int rt_mutex_getprio(struct task_struct *p);
 extern void rt_mutex_setprio(struct task_struct *p, int prio);
 extern int rt_mutex_get_effective_prio(struct task_struct *task, int newprio);
+extern void rt_mutex_update_top_task(struct task_struct *p);
 extern struct task_struct *rt_mutex_get_top_task(struct task_struct *task);
 extern void rt_mutex_adjust_pi(struct task_struct *p);
 static inline bool tsk_is_pi_blocked(struct task_struct *tsk)

diff --git a/kernel/fork.c b/kernel/fork.c
index 6c463c80e93de8c3be3180f3cbd8694b955a1ac3..b30196a00b0de5acbdce4bfc09d007c61dd0e4d0 100644 (file)
--- 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 71ecf0624410c05ce2b39d678fe337748192ec0f..bc05b104eaed32768195b5537924303246a192f4 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -322,6 +322,19 @@ rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter)
        RB_CLEAR_NODE(&waiter->pi_tree_entry);
 }
 
+/*
+ * 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
  *
@@ -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 ab9f6ac099a71368ccb22c857c82239e2183fb28..e1f44ec701c835633ed1f03d8bb3b16d63c0b4fa 100644 (file)
--- 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;