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Date:   Fri, 27 Sep 2019 13:26:14 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        linux-kernel@vger.kernel.org
Subject: Re: Regression in dbdda842fe96 ("printk: Add console owner and
 waiter logic to load balance console writes") [Was: Regression in
 fd5f7cde1b85 ("...")]
Message-ID: <20190927042614.GA784@jagdpanzerIV>
References: <20190917141034.gvjg7bgylqbbxyv7@pengutronix.de>
 <20190918013032.GA2895@jagdpanzerIV>
 <20190918071158.rtw45jch2roa2wum@pengutronix.de>
 <20190918075252.GA30808@jagdpanzerIV>
 <20190926085855.debu7t46s7kgb26p@pathway.suse.cz>
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On (09/26/19 10:58), Petr Mladek wrote:
[..]
> > -	spin_lock(&sport->port.lock);
> > -
> > +	uart_port_lock_irqsave(&sport->port, flags);
> 
> uart_port_lock_irqsave() does not exist.

... Oh. Good catch! Apparently I still carry around my patch set
which added printk_safe to TTY/UART locking API.

> Instead the current users do:
> 
>      spin_lock_irqsave(&port->lock, flags);

Right.

[..]

> I like this approach. It allows to remove hacks with locks.

[..]

> Or I would keep the locking as is and add some API
> just for the sysrq handling:
>
>
>    int uart_store_sysrq_char(struct uart_port *port, unsigned int ch);
>    unsigned int uart_get_sysrq_char(struct uart_port *port);

Looks good. We also probably can remove struct uart_port's
->sysrq member and clean up locking in drivers' ->write()
callbacks:

	if (sport->sysrq)
		locked = 0;
	else if (oops_in_progress)
		locked = spin_trylock_irqsave(&sport->lock, flags);
	else
		spin_lock_irqsave(&sport->lock, flags);

Because this ->sysrq branch makes driver completely lockless globally,
for all CPUs, not only for sysrq-CPU.

> And use it the following way:
> 
> 	int handle_irq()
> 	{
> 		unsined int sysrq, sysrq_ch;
> 
> 		spin_lock(&port->lock);
> 		[...]
> 			sysrq = uart_store_sysrq_char(port, ch);
> 			if (!sysrq)
> 				[...]
> 		[...]
> 
> 	out:
> 		sysrq_ch = uart_get_sysrq_char(port);
> 		spin_unlock(&port->lock);
> 
> 		if (sysrq_ch)
> 			handle_sysrq(sysrq_ch);
> 	}

Looks good.

	-ss