rtmutex: Deboost before waking up the top waiter

We should deboost before waking the high-priority task, such that we
don't run two tasks with the same "state" (priority, deadline,
sched_class, etc).

In order to make sure the boosting task doesn't start running between
unlock and deboost (due to 'spurious' wakeup), we move the deboost
under the wait_lock, that way its serialized against the wait loop in
__rt_mutex_slowlock().

Doing the deboost early can however lead to priority-inversion if
current would get preempted after the deboost but before waking our
high-prio task, hence we disable preemption before doing deboost, and
enabling it after the wake up is over.

This gets us the right semantic order, but most importantly however;
this change ensures pointer stability for the next patch, where we
have rt_mutex_setprio() cache a pointer to the top-most waiter task.
If we, as before this change, do the wakeup first and then deboost,
this pointer might point into thin air.

[peterz: Changelog + patch munging]
Suggested-by: 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>
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/20170323150216.110065320@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/futex.c b/kernel/futex.c
index 628be42296ebf31aafbf839633c4e8e2c01715d0..414a30dbc6b659f3580bddbfa7af7b790a63b2f7 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1460,10 +1460,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_
 out_unlock:
        raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 
-       if (deboost) {
-               wake_up_q(&wake_q);
-               rt_mutex_adjust_prio(current);
-       }
+       rt_mutex_postunlock(&wake_q, deboost);
 
        return ret;
 }

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index dd103124166be2562e26b60fdaab1e2d773eff81..71ecf0624410c05ce2b39d678fe337748192ec0f 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -372,24 +372,6 @@ static void __rt_mutex_adjust_prio(struct task_struct *task)
                rt_mutex_setprio(task, prio);
 }
 
-/*
- * 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:
  *
@@ -1051,6 +1033,7 @@ 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
@@ -1393,6 +1376,16 @@ static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
         */
        mark_wakeup_next_waiter(wake_q, lock);
 
+       /*
+        * We should deboost before waking the top waiter 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. Pairs with preempt_enable() in
+        * rt_mutex_postunlock();
+        */
+       preempt_disable();
+
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 
        /* check PI boosting */
@@ -1442,6 +1435,18 @@ rt_mutex_fasttrylock(struct rt_mutex *lock,
        return slowfn(lock);
 }
 
+/*
+ * Undo pi boosting (if necessary) and wake top waiter.
+ */
+void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
+{
+       wake_up_q(wake_q);
+
+       /* Pairs with preempt_disable() in rt_mutex_slowunlock() */
+       if (deboost)
+               preempt_enable();
+}
+
 static inline void
 rt_mutex_fastunlock(struct rt_mutex *lock,
                    bool (*slowfn)(struct rt_mutex *lock,
@@ -1455,11 +1460,7 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
 
        deboost = slowfn(lock, &wake_q);
 
-       wake_up_q(&wake_q);
-
-       /* Undo pi boosting if necessary: */
-       if (deboost)
-               rt_mutex_adjust_prio(current);
+       rt_mutex_postunlock(&wake_q, deboost);
 }
 
 /**
@@ -1572,6 +1573,13 @@ bool __sched __rt_mutex_futex_unlock(struct rt_mutex *lock,
        }
 
        mark_wakeup_next_waiter(wake_q, lock);
+       /*
+        * We've already deboosted, retain preempt_disabled when dropping
+        * the wait_lock to avoid inversion until the wakeup. Matched
+        * by rt_mutex_postunlock();
+        */
+       preempt_disable();
+
        return true; /* deboost and wakeups */
 }
 
@@ -1584,10 +1592,7 @@ void __sched rt_mutex_futex_unlock(struct rt_mutex *lock)
        deboost = __rt_mutex_futex_unlock(lock, &wake_q);
        raw_spin_unlock_irq(&lock->wait_lock);
 
-       if (deboost) {
-               wake_up_q(&wake_q);
-               rt_mutex_adjust_prio(current);
-       }
+       rt_mutex_postunlock(&wake_q, deboost);
 }
 
 /**

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index b1ccfea2effe8683f8be147710ac0c84a0f00fe9..a09c02982391a41b7283b80ebfb342db9a72ed34 100644 (file)
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -122,7 +122,7 @@ extern void rt_mutex_futex_unlock(struct rt_mutex *lock);
 extern bool __rt_mutex_futex_unlock(struct rt_mutex *lock,
                                 struct wake_q_head *wqh);
 
-extern void rt_mutex_adjust_prio(struct task_struct *task);
+extern void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost);
 
 #ifdef CONFIG_DEBUG_RT_MUTEXES
 # include "rtmutex-debug.h"