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Date:   Thu, 2 May 2019 12:09:32 +0200
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)
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On 05/01, Peter Zijlstra wrote:
>
> Anyway; I cobbled together the below. Oleg, could you have a look, I'm
> sure I messed it up.

Oh, I will need to read this carefully. but at first glance I do not see
any hole...

> +static void readers_block(struct percpu_rw_semaphore *sem)
> +{
> +	wait_event_cmd(sem->writer, !sem->readers_block,
> +		       __up_read(&sem->rw_sem), __down_read(&sem->rw_sem));
> +}
> +
> +static void block_readers(struct percpu_rw_semaphore *sem)
> +{
> +	wait_event_exclusive_cmd(sem->writer, !sem->readers_block,
> +				 __up_write(&sem->rw_sem),
> +				 __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);
> +}

So iiuc, despite it name block_readers() also serializes the writers, ->rw_sem
can be dropped by down_write_non_owner() so the new writer can take this lock.

And note that the caller of readers_block() does down_read(), the caller of
block_readers() does down_write(). So perhaps it makes sense to shift these
down_read/write into the helpers above and rename them,

	void xxx_down_read(struct percpu_rw_semaphore *sem)
	{
		__down_read(&sem->rw_sem);

		wait_event_cmd(sem->writer, !sem->readers_block,
		       __up_read(&sem->rw_sem), __down_read(&sem->rw_sem));
	}

	void xxx_down_write(struct percpu_rw_semaphore *sem)
	{
		down_write(&sem->rw_sem);

		wait_event_exclusive_cmd(sem->writer, !sem->readers_block,
					 __up_write(&sem->rw_sem),
					 __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);
	}

to make this logic more clear? Or even

	bool ck_read(struct percpu_rw_semaphore *sem)
	{
		__down_read(&sem->rw_sem);
		if (!sem->readers_block)
			return true;
		__up_read(&sem->rw_sem);
	}

	bool ck_write(struct percpu_rw_semaphore *sem)
	{
		down_write(&sem->rw_sem);
		if (!sem->readers_block)
			return true;
		up_write(&sem->rw_sem);
	}

Then percpu_down_read/write can simply do wait_event(ck_read(sem)) and
wait_event_exclusive(ck_write(sem)) respectively.

But this all is cosmetic, it seems that we can remove ->rw_sem altogether
but I am not sure...

Oleg.