From: Xunlei Pang xlpang@redhat.com
commit 2a1c6029940675abb2217b590512dbf691867ec4 upstream.
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 Tested-by: Henrik Austad haustad@cisco.com --- kernel/futex.c | 5 +--- kernel/locking/rtmutex.c | 59 ++++++++++++++++++++++------------------- kernel/locking/rtmutex_common.h | 2 +- 3 files changed, 34 insertions(+), 32 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index afb02a7..63fa840 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1457,10 +1457,7 @@ 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 b061a79..c01d7f4 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -371,24 +371,6 @@ 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 @@ -1049,6 +1031,7 @@ 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 @@ -1391,6 +1374,16 @@ 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 */ @@ -1440,6 +1433,18 @@ 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, @@ -1453,11 +1458,7 @@ 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); }
/** @@ -1570,6 +1571,13 @@ 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 */ }
@@ -1582,10 +1590,7 @@ 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 25ccf71..6f07357 100644 --- 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"