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Commit 2a1c6029 authored by Xunlei Pang's avatar Xunlei Pang Committed by Thomas Gleixner
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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: default avatarPeter 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>
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: default avatarThomas Gleixner <tglx@linutronix.de>
parent 38bffdac

kernel/futex.c

+1 −4
Original line number Diff line number Diff line
@@ -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;

}

kernel/locking/rtmutex.c

+32 −27
Original line number Diff line number Diff line
@@ -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);

}



/**

kernel/locking/rtmutex_common.h

+1 −1
Original line number Diff line number Diff line
@@ -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"