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Commit e96a7705 authored by Xunlei Pang's avatar Xunlei Pang Committed by Thomas Gleixner
Browse files

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: default avatarXunlei Pang <xlpang@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Reviewed-by: default avatarThomas 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent 2a1c6029

include/linux/init_task.h

+1 −0
Original line number Original line Diff line number Diff line
@@ -181,6 +181,7 @@ extern struct cred init_cred; 
#ifdef CONFIG_RT_MUTEXES

#ifdef CONFIG_RT_MUTEXES

# define INIT_RT_MUTEXES(tsk)						\

# define INIT_RT_MUTEXES(tsk)						\

	.pi_waiters = RB_ROOT,						\

	.pi_waiters = RB_ROOT,						\

	.pi_top_task = NULL,						\

	.pi_waiters_leftmost = NULL,

	.pi_waiters_leftmost = NULL,

#else

#else

# define INIT_RT_MUTEXES(tsk)

# define INIT_RT_MUTEXES(tsk)

include/linux/sched.h

+2 −0
Original line number Original line Diff line number Diff line
@@ -775,6 +775,8 @@ struct task_struct { 
	/* PI waiters blocked on a rt_mutex held by this task: */

	/* PI waiters blocked on a rt_mutex held by this task: */

	struct rb_root			pi_waiters;

	struct rb_root			pi_waiters;

	struct rb_node			*pi_waiters_leftmost;

	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: */

	/* Deadlock detection and priority inheritance handling: */

	struct rt_mutex_waiter		*pi_blocked_on;

	struct rt_mutex_waiter		*pi_blocked_on;

#endif

#endif

include/linux/sched/rt.h

+1 −0
Original line number Original line Diff line number Diff line
@@ -21,6 +21,7 @@ static inline int rt_task(struct task_struct *p) 
extern int rt_mutex_getprio(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 void rt_mutex_setprio(struct task_struct *p, int prio);

extern int rt_mutex_get_effective_prio(struct task_struct *task, int newprio);

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 struct task_struct *rt_mutex_get_top_task(struct task_struct *task);

extern void rt_mutex_adjust_pi(struct task_struct *p);

extern void rt_mutex_adjust_pi(struct task_struct *p);

static inline bool tsk_is_pi_blocked(struct task_struct *tsk)

static inline bool tsk_is_pi_blocked(struct task_struct *tsk)

kernel/fork.c

+1 −0
Original line number Original line Diff line number Diff line
@@ -1438,6 +1438,7 @@ static void rt_mutex_init_task(struct task_struct *p) 
#ifdef CONFIG_RT_MUTEXES

#ifdef CONFIG_RT_MUTEXES

	p->pi_waiters = RB_ROOT;

	p->pi_waiters = RB_ROOT;

	p->pi_waiters_leftmost = NULL;

	p->pi_waiters_leftmost = NULL;

	p->pi_top_task = NULL;

	p->pi_blocked_on = NULL;

	p->pi_blocked_on = NULL;

#endif

#endif

}

}

kernel/locking/rtmutex.c

+21 −8
Original line number Original line Diff line number Diff line
@@ -322,6 +322,19 @@ rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter) 
	RB_CLEAR_NODE(&waiter->pi_tree_entry);

	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

 * Calculate task priority from the waiter tree priority

 *

 *
@@ -337,12 +350,12 @@ int rt_mutex_getprio(struct task_struct *task) 
		   task->normal_prio);

		   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)

struct task_struct *rt_mutex_get_top_task(struct task_struct *task)

{

{

	if (likely(!task_has_pi_waiters(task)))

	return task->pi_top_task;

		return NULL;



	return task_top_pi_waiter(task)->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)

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);

		return newprio;





	if (task_top_pi_waiter(task)->task->prio <= newprio)

	if (!top_task)

		return task_top_pi_waiter(task)->task->prio;

		return newprio;

		return newprio;



	return min(top_task->prio, newprio);

}

}





/*

/*