linux-kernel - Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

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Date:   Wed, 28 Sep 2016 11:07:40 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        rostedt@...dmis.org, xlpang@...hat.com,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

On 2016-09-26 14:32:14 [+0200], Peter Zijlstra wrote:
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1374,9 +1374,8 @@ static int wake_futex_pi(u32 __user *uad
>  	 * scheduled away before the wake up can take place.
>  	 */
>  	spin_unlock(&hb->lock);
> -	wake_up_q(&wake_q);
> -	if (deboost)
> -		rt_mutex_adjust_prio(current);
> +
> +	rt_mutex_postunlock(&wake_q, deboost);

This breaks -RT. Before that spin_unlock() you do a preempt_disable()
which means you had one spinlock with enabled preemption and now you get
one unlock with disabled preemption. And this breaks migrate_disable() /
enable (because we take the fast path in the in_atomic() case).

>  	return 0;
>  }
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -307,24 +307,6 @@ static void __rt_mutex_adjust_prio(struc
>  }
>  
>  /*
> - * 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);
> -}

I don't see this function getting back somewhere in this patch. There is
one occurrence left in kernel/locking/rtmutex_common.h

Sebastian