[PATCH v3 2/6] sched/rtmutex/deadline: Fix a PI crash for deadline tasks
From: Xunlei Pang
Date: Thu Apr 14 2016 - 07:37:43 EST
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:
[<ffffffff810cf8aa>] ? enqueue_task_dl+0x2a/0x320
[<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
[<ffffffff810cfa15>] ? enqueue_task_dl+0x195/0x320
[<ffffffff810d0bac>] ? prio_changed_dl+0x6c/0x90
[<ffffffff810ed8b0>] do_futex+0x190/0x5b0
[<ffffffff810edd50>] SyS_futex+0x80/0x180
[<ffffffff8165a089>] system_call_fastpath+0x16/0x1b
RIP [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
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()(now replaced by rt_mutex_get_top_waiter()
by previous patches, same issue), will access them with rq lock
held but not holding pi_lock.
In order to tackle it, we introduce new "pi_waiters_leftmost_copy"
in task_struct, and add a new function named rt_mutex_update_copy()
to do the copy, it can be called by rt_mutex_setprio() which held
owner's pi_lock, rq lock, and rt_mutex lock. But one exception is
that rt_mutex_setprio() can be called without rtmutex locked after
mark_wakeup_next_waiter() by rt_mutex_adjust_prio(), so we need a
new function in mark_wakeup_next_waiter() to lock rq first then
call rt_mutex_update_copy().
We avoid adding new logic by calling __rt_mutex_adjust_prio() directly
in mark_wakeup_next_waiter()and remove the old rt_mutex_adjust_prio()
outside the lock. Since we moved the deboost point, in order to avoid
current process to be preempted(due to deboost earlier) before finishing
wake_up_q(), we also move preempt_disable() before unlocking rtmutex.
Originally-From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxx>
---
include/linux/init_task.h | 3 ++-
include/linux/sched.h | 3 +++
include/linux/sched/deadline.h | 2 ++
kernel/fork.c | 1 +
kernel/locking/rtmutex.c | 61 +++++++++++++++++-------------------------
kernel/sched/core.c | 2 ++
6 files changed, 35 insertions(+), 37 deletions(-)
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index f2cb8d4..7be5a83 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -162,7 +162,8 @@ extern struct task_group root_task_group;
#ifdef CONFIG_RT_MUTEXES
# define INIT_RT_MUTEXES(tsk) \
.pi_waiters = RB_ROOT, \
- .pi_waiters_leftmost = NULL,
+ .pi_waiters_leftmost = NULL, \
+ .pi_waiters_leftmost_copy = NULL,
#else
# define INIT_RT_MUTEXES(tsk)
#endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 45e848c..332a6b5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1621,7 +1621,10 @@ struct task_struct {
#ifdef CONFIG_RT_MUTEXES
/* PI waiters blocked on a rt_mutex held by this task */
struct rb_root pi_waiters;
+ /* Updated under pi_lock and rtmutex lock */
struct rb_node *pi_waiters_leftmost;
+ /* Updated under pi_lock, rq_lock, and rtmutex lock */
+ struct rb_node *pi_waiters_leftmost_copy;
/* Deadlock detection and priority inheritance handling */
struct rt_mutex_waiter *pi_blocked_on;
#endif
diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
index 9089a2a..e8304d4 100644
--- a/include/linux/sched/deadline.h
+++ b/include/linux/sched/deadline.h
@@ -26,4 +26,6 @@ static inline bool dl_time_before(u64 a, u64 b)
return (s64)(a - b) < 0;
}
+extern void rt_mutex_update_copy(struct task_struct *p);
+
#endif /* _SCHED_DEADLINE_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index a453546..4b847e4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1225,6 +1225,7 @@ 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_waiters_leftmost_copy = NULL;
p->pi_blocked_on = NULL;
#endif
}
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 42bc59b..00c6560 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -273,10 +273,11 @@ int rt_mutex_getprio(struct task_struct *task)
struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
{
- if (likely(!task_has_pi_waiters(task)))
+ if (!task->pi_waiters_leftmost_copy)
return NULL;
- return task_top_pi_waiter(task)->task;
+ return rb_entry(task->pi_waiters_leftmost_copy,
+ struct rt_mutex_waiter, pi_tree_entry)->task;
}
/*
@@ -285,12 +286,20 @@ 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);
+}
+
+/*
+ * Must hold rq lock, p's pi_lock, and rt_mutex lock.
+ */
+void rt_mutex_update_copy(struct task_struct *p)
+{
+ p->pi_waiters_leftmost_copy = p->pi_waiters_leftmost;
}
/*
@@ -307,24 +316,6 @@ static void __rt_mutex_adjust_prio(struct task_struct *task)
}
/*
- * Adjust task priority (undo boosting). Called from the exit path of
- * rt_mutex_slowunlock() and rt_mutex_slowlock().
- *
- * (Note: We do this outside of the protection of lock->wait_lock to
- * allow the lock to be taken while or before we readjust the priority
- * of task. We do not use the spin_xx_mutex() variants here as we are
- * outside of the debug path.)
- */
-void rt_mutex_adjust_prio(struct task_struct *task)
-{
- unsigned long flags;
-
- raw_spin_lock_irqsave(&task->pi_lock, flags);
- __rt_mutex_adjust_prio(task);
- raw_spin_unlock_irqrestore(&task->pi_lock, flags);
-}
-
-/*
* Deadlock detection is conditional:
*
* If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
@@ -987,6 +978,7 @@ static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
* lock->wait_lock.
*/
rt_mutex_dequeue_pi(current, waiter);
+ __rt_mutex_adjust_prio(current);
/*
* As we are waking up the top waiter, and the waiter stays
@@ -1325,6 +1317,15 @@ static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
*/
mark_wakeup_next_waiter(wake_q, lock);
+ /*
+ * We should deboost before waking the high-prio task such that
+ * we don't run two tasks with the 'same' priority. This however
+ * can lead to prio-inversion if we would get preempted after
+ * the deboost but before waking our high-prio task, hence the
+ * preempt_disable before unlock.
+ */
+ preempt_disable();
+
raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
/* check PI boosting */
@@ -1400,18 +1401,6 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
*/
void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
{
- /*
- * We should deboost before waking the high-prio task such that
- * we don't run two tasks with the 'same' priority. This however
- * can lead to prio-inversion if we would get preempted after
- * the deboost but before waking our high-prio task, hence the
- * preempt_disable.
- */
- if (deboost) {
- preempt_disable();
- rt_mutex_adjust_prio(current);
- }
-
wake_up_q(wake_q);
if (deboost)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1159423..4a2c79d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3480,6 +3480,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
goto out_unlock;
}
+ rt_mutex_update_copy(p);
+
trace_sched_pi_setprio(p, prio);
oldprio = p->prio;
--
1.8.3.1