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From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Clark Williams <williams@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	jack@suse.com, Waiman Long <longman@redhat.com>,
	Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)
Date: Fri, 3 May 2019 17:25:13 +0200
Message-ID: <20190503152513.GE2650@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190503145059.GC2606@hirez.programming.kicks-ass.net>

On Fri, May 03, 2019 at 04:50:59PM +0200, Peter Zijlstra wrote:
> So how about something like so then?

> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c

> @@ -63,7 +66,7 @@ int __percpu_down_read(struct percpu_rw_
>  	 * If !readers_block the critical section starts here, matched by the
>  	 * release in percpu_up_write().
>  	 */
> -	if (likely(!smp_load_acquire(&sem->readers_block)))
> +	if (likely(!atomic_read_acquire(&sem->block)))
>  		return 1;
>  
>  	/*
> @@ -80,14 +83,8 @@ int __percpu_down_read(struct percpu_rw_
>  	 * and reschedule on the preempt_enable() in percpu_down_read().
>  	 */
>  	preempt_enable_no_resched();
> -
> -	/*
> -	 * Avoid lockdep for the down/up_read() we already have them.
> -	 */
> -	__down_read(&sem->rw_sem);
> +	wait_event(sem->waiters, !atomic_read(&sem->block));

That should be:

	wait_event(sem->waiters, !atomic_read_acquire(&sem->block));

I suppose.

>  	this_cpu_inc(*sem->read_count);
> -	__up_read(&sem->rw_sem);
> -
>  	preempt_disable();
>  	return 1;
>  }
> @@ -104,7 +101,7 @@ void __percpu_up_read(struct percpu_rw_s
>  	__this_cpu_dec(*sem->read_count);
>  
>  	/* Prod writer to recheck readers_active */
> -	rcuwait_wake_up(&sem->writer);
> +	wake_up(&sem->waiters);
>  }
>  EXPORT_SYMBOL_GPL(__percpu_up_read);
>  
> @@ -139,18 +136,22 @@ static bool readers_active_check(struct
>  	return true;
>  }
>  
> +static inline bool acquire_block(struct percpu_rw_semaphore *sem)
> +{
> +	if (atomic_read(&sem->block))
> +		return false;
> +
> +	return atomic_xchg(&sem->block, 1) == 0;
> +}
> +
>  void percpu_down_write(struct percpu_rw_semaphore *sem)
>  {
> +	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
> +
>  	/* Notify readers to take the slow path. */
>  	rcu_sync_enter(&sem->rss);
>  
> -	down_write(&sem->rw_sem);
> -
> -	/*
> -	 * Notify new readers to block; up until now, and thus throughout the
> -	 * longish rcu_sync_enter() above, new readers could still come in.
> -	 */
> -	WRITE_ONCE(sem->readers_block, 1);
> +	wait_event_exclusive(sem->waiters, acquire_block(sem));
>  
>  	smp_mb(); /* D matches A */

And we can remove that smp_mb() and rely on the atomic_xchg() from
acquire_block().

>  
> @@ -161,7 +162,7 @@ void percpu_down_write(struct percpu_rw_
>  	 */
>  
>  	/* Wait for all now active readers to complete. */
> -	rcuwait_wait_event(&sem->writer, readers_active_check(sem));
> +	wait_event(sem->waiters, readers_active_check(sem));
>  }
>  EXPORT_SYMBOL_GPL(percpu_down_write);
>  
> @@ -177,12 +178,8 @@ void percpu_up_write(struct percpu_rw_se
>  	 * Therefore we force it through the slow path which guarantees an
>  	 * acquire and thereby guarantees the critical section's consistency.
>  	 */
> -	smp_store_release(&sem->readers_block, 0);
> -
> -	/*
> -	 * Release the write lock, this will allow readers back in the game.
> -	 */
> -	up_write(&sem->rw_sem);
> +	atomic_set_release(&sem->block, 0);
> +	wake_up(&sem->waiters);
>  
>  	/*
>  	 * Once this completes (at least one RCU-sched grace period hence) the
> @@ -190,5 +187,21 @@ void percpu_up_write(struct percpu_rw_se
>  	 * exclusive write lock because its counting.
>  	 */
>  	rcu_sync_exit(&sem->rss);
> +
> +	rwsem_release(&sem->dep_map, 1, _RET_IP_);
>  }
>  EXPORT_SYMBOL_GPL(percpu_up_write);

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-26  9:34 Juri Lelli
2019-03-28 10:17 ` Sebastian Andrzej Siewior
2019-04-19  8:56 ` Juri Lelli
2019-04-30 12:51   ` Sebastian Andrzej Siewior
2019-04-30 13:28     ` Peter Zijlstra
2019-04-30 13:45       ` Sebastian Andrzej Siewior
2019-04-30 14:01         ` Peter Zijlstra
2019-04-30 14:15       ` Oleg Nesterov
2019-04-30 14:29         ` Peter Zijlstra
2019-04-30 14:42         ` Oleg Nesterov
2019-04-30 14:44           ` Peter Zijlstra
2019-04-30 14:53             ` Oleg Nesterov
2019-05-01 17:09       ` Peter Zijlstra
2019-05-01 17:26         ` Waiman Long
2019-05-01 18:54           ` Peter Zijlstra
2019-05-01 19:22             ` Davidlohr Bueso
2019-05-01 19:25               ` Peter Zijlstra
2019-05-02 10:09         ` Oleg Nesterov
2019-05-02 11:42           ` Oleg Nesterov
2019-05-03 14:50             ` Peter Zijlstra
2019-05-03 15:25               ` Peter Zijlstra [this message]
2019-05-06 16:50               ` Oleg Nesterov
2019-06-19  9:50                 ` Peter Zijlstra
2019-05-03 14:16           ` Peter Zijlstra
2019-05-03 15:37             ` Oleg Nesterov
2019-05-03 15:46               ` Peter Zijlstra

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