On Mon, 26 Sep 2016 14:32:18 +0200
Peter Zijlstra <pet...@infradead.org> wrote:
> Previous patches changed the meaning of the return value of
> rt_mutex_slowunlock(); update comments and code to reflect this.
>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> kernel/futex.c | 12 ++++++------
> kernel/locking/rtmutex.c | 20 +++++++++-----------
> kernel/locking/rtmutex_common.h | 2 +-
> 3 files changed, 16 insertions(+), 18 deletions(-)
>
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1261,7 +1261,7 @@ static int wake_futex_pi(u32 __user *uad
> struct futex_pi_state *pi_state = this->pi_state;
> u32 uninitialized_var(curval), newval;
> WAKE_Q(wake_q);
> - bool deboost;
> + bool postunlock;
> int ret = 0;
>
> if (!pi_state)
> @@ -1327,17 +1327,17 @@ static int wake_futex_pi(u32 __user *uad
>
> raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
>
> - deboost = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
> + postunlock = rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
>
> /*
> * First unlock HB so the waiter does not spin on it once he got woken
> - * up. Second wake up the waiter before the priority is adjusted. If we
> - * deboost first (and lose our higher priority), then the task might get
> - * scheduled away before the wake up can take place.
> + * up. Then wakeup the waiter by calling rt_mutex_postunlock(). Priority
> + * is already adjusted and preemption is disabled to avoid inversion.
Can we specify here that preemption is only disabled if
rt_mutex_futex_unlock() returns true, and will be enabled again with
rt_mutex_postunlock().
> */
> spin_unlock(&hb->lock);
>
> - rt_mutex_postunlock(&wake_q, deboost);
> + if (postunlock)
> + rt_mutex_postunlock(&wake_q);
>
> return 0;
> }
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1254,7 +1254,8 @@ static inline int rt_mutex_slowtrylock(s
>
> /*
> * Slow path to release a rt-mutex.
> - * Return whether the current task needs to undo a potential priority
> boosting.
> + *
> + * Return whether the current task needs to call rt_mutex_postunlock().
> */
> static bool __sched rt_mutex_slowunlock(struct rt_mutex *lock,
> struct wake_q_head *wake_q)
> @@ -1327,7 +1328,7 @@ static bool __sched rt_mutex_slowunlock(
>
> raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>
> - /* check PI boosting */
> + /* call rt_mutex_postunlock() */
Can we rephrase this to "A call to rt_mutex_postunlock() is required".
> return true;
> }
>
> @@ -1378,15 +1379,14 @@ rt_mutex_fasttrylock(struct rt_mutex *lo
> }
>
> /*
> - * Undo pi boosting (if necessary) and wake top waiter.
> + * Performs the wakeup of the the top-waiter and re-enables preemption.
> */
> -void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
> +void rt_mutex_postunlock(struct wake_q_head *wake_q)
> {
> wake_up_q(wake_q);
>
> /* Pairs with preempt_disable() in rt_mutex_slowunlock() */
> - if (deboost)
> - preempt_enable();
> + preempt_enable();
> }
>
> /**
> @@ -1489,9 +1489,8 @@ void __sched rt_mutex_unlock(struct rt_m
> rt_mutex_deadlock_account_unlock(current);
>
> } else {
> - bool deboost = rt_mutex_slowunlock(lock, &wake_q);
> -
> - rt_mutex_postunlock(&wake_q, deboost);
> + if (rt_mutex_slowunlock(lock, &wake_q))
> + rt_mutex_postunlock(&wake_q);
> }
> }
> EXPORT_SYMBOL_GPL(rt_mutex_unlock);
> @@ -1500,8 +1499,7 @@ EXPORT_SYMBOL_GPL(rt_mutex_unlock);
> * rt_mutex_futex_unlock - Futex variant of rt_mutex_unlock
> * @lock: the rt_mutex to be unlocked
> *
> - * Returns: true/false indicating whether priority adjustment is
> - * required or not.
> + * Returns: true/false indicating whether we should call
> rt_mutex_postunlock().
Can this be rephrased to: "Returns true if preemption has been
disabled and a call to rt_mutex_postunlock() is required (which will
re-enable preemption)"
-- Steve
> */
> bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
> struct wake_q_head *wqh)
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -111,7 +111,7 @@ extern int rt_mutex_finish_proxy_lock(st
> extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct
> hrtimer_sleeper *to);
> extern bool rt_mutex_futex_unlock(struct rt_mutex *lock,
> struct wake_q_head *wqh);
> -extern void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost);
> +extern void rt_mutex_postunlock(struct wake_q_head *wake_q);
> extern void rt_mutex_adjust_prio(struct task_struct *task);
>
> #ifdef CONFIG_DEBUG_RT_MUTEXES
>
