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From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
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: Mon, 6 May 2019 18:50:09 +0200
Message-ID: <20190506165009.GA28959@redhat.com> (raw)
In-Reply-To: <20190503145059.GC2606@hirez.programming.kicks-ass.net>

On 05/03, Peter Zijlstra wrote:
>
> -static void lockdep_sb_freeze_release(struct super_block *sb)
> -{
> -	int level;
> -
> -	for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
> -		percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> -}
> -
> -/*
> - * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
> - */
> -static void lockdep_sb_freeze_acquire(struct super_block *sb)
> -{
> -	int level;
> -
> -	for (level = 0; level < SB_FREEZE_LEVELS; ++level)
> -		percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
> +	percpu_down_write_non_owner(sb->s_writers.rw_sem + level-1);
>  }

I'd suggest to not change fs/super.c, keep these helpers, and even not introduce
xxx_write_non_owner().

freeze_super() takes other locks, it calls sync_filesystem(), freeze_fs(), lockdep
should know that this task holds SB_FREEZE_XXX locks for writing.


> @@ -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));
>  	this_cpu_inc(*sem->read_count);

Argh, this looks racy :/

Suppose that sem->block == 0 when wait_event() is called, iow the writer released
the lock.

Now suppose that this __percpu_down_read() races with another percpu_down_write().
The new writer can set sem->block == 1 and call readers_active_check() in between,
after wait_event() and before this_cpu_inc(*sem->read_count).

Oleg.


  parent 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
2019-05-06 16:50               ` Oleg Nesterov [this message]
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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