linux-kernel - Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait

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Date:   Tue, 14 Sep 2021 00:08:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 1/4] sched/wakeup: Strengthen
 current_save_and_set_rtlock_wait_state()

On Thu, Sep 09 2021 at 12:59, Peter Zijlstra wrote:
> While looking at current_save_and_set_rtlock_wait_state() I'm thinking
> it really ought to use smp_store_mb(), because something like:
>
> 	current_save_and_set_rtlock_wait_state();
> 	for (;;) {
> 		if (try_lock())
> 			break;
>
> 		raw_spin_unlock_irq(&lock->wait_lock);
> 		schedule();
> 		raw_spin_lock_irq(&lock->wait_lock);
>
> 		set_current_state(TASK_RTLOCK_WAIT);
> 	}
> 	current_restore_rtlock_saved_state();
>
> which is the advertised usage in the comment, is actually broken,
> since trylock() will only need a load-acquire in general and that
> could be re-ordered against the state store, which could lead to a
> missed wakeup -> BAD (tm).

I don't think so because both the state store and the wakeup are
serialized via tsk->pi_lock.

> While there, make them consistent with the IRQ usage in
> set_special_state().
>
> Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/sched.h |   19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -245,7 +245,8 @@ struct task_group;
>   *		if (try_lock())
>   *			break;
>   *		raw_spin_unlock_irq(&lock->wait_lock);
> - *		schedule_rtlock();
> + *		if (!cond)
> + *			schedule_rtlock();

cond is not really relevant here.

>   *		raw_spin_lock_irq(&lock->wait_lock);
>   *		set_current_state(TASK_RTLOCK_WAIT);
>   *	}
> @@ -253,22 +254,24 @@ struct task_group;
>   */
>  #define current_save_and_set_rtlock_wait_state()			\
>  	do {								\
> -		lockdep_assert_irqs_disabled();				\
> -		raw_spin_lock(&current->pi_lock);			\
> +		unsigned long flags; /* may shadow */			\
> +									\
> +		raw_spin_lock_irqsave(&current->pi_lock, flags);	\

Why? This is solely for the rtlock use case which invokes this with
interrupts disabled. So why do we need that irqsave() overhead here?

>  		current->saved_state = current->__state;		\
>  		debug_rtlock_wait_set_state();				\
> -		WRITE_ONCE(current->__state, TASK_RTLOCK_WAIT);		\
> -		raw_spin_unlock(&current->pi_lock);			\
> +		smp_store_mb(current->__state, TASK_RTLOCK_WAIT);	\

The try_lock() does not matter at all here, really. All what matters is
that the unlocker cannot observe the wrong state and that's fully
serialized via tsk::pi_lock.

Thanks,

        tglx