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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>
hifive-unleashed-5.1
Xunlei Pang 2017-03-23 15:56:07 +01:00 committed by Thomas Gleixner
parent 38bffdac07
commit 2a1c602994
3 changed files with 34 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
kernel/futex.c
View File

@ -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;
}

59
kernel/locking/rtmutex.c
View File

@ -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);
}
/**

2
kernel/locking/rtmutex_common.h
View File

@ -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"