linux-kernel - Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)

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Date:   Mon, 6 May 2019 18:50:09 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>, jack@...e.com,
        Waiman Long <longman@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)

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.