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

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.orgSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 2a1c6029
...@@ -181,6 +181,7 @@ extern struct cred init_cred; ...@@ -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)
......
...@@ -775,6 +775,8 @@ struct task_struct { ...@@ -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
......
...@@ -21,6 +21,7 @@ static inline int rt_task(struct task_struct *p) ...@@ -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)
......
...@@ -1438,6 +1438,7 @@ static void rt_mutex_init_task(struct task_struct *p) ...@@ -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
} }
......
...@@ -322,6 +322,19 @@ rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter) ...@@ -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) ...@@ -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) ...@@ -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);
if (!top_task)
return newprio; return newprio;
if (task_top_pi_waiter(task)->task->prio <= newprio) return min(top_task->prio, newprio);
return task_top_pi_waiter(task)->task->prio;
return newprio;
} }
/* /*
......
...@@ -3713,6 +3713,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio) ...@@ -3713,6 +3713,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
goto out_unlock; goto out_unlock;
} }
rt_mutex_update_top_task(p);
trace_sched_pi_setprio(p, prio); trace_sched_pi_setprio(p, prio);
oldprio = p->prio; oldprio = p->prio;
......
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