linux-kernel - Re: 2.6.18-rt1

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Date:	Fri, 22 Sep 2006 07:42:50 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Gene Heskett <gene.heskett@...izon.net>,
	linux-kernel@...r.kernel.org, paulmck@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: 2.6.18-rt1

On Thu, 2006-09-21 at 21:02 +0200, Ingo Molnar wrote:
> * Daniel Walker <dwalker@...sta.com> wrote:
> 
> > On Wed, 2006-09-20 at 22:14 +0200, Ingo Molnar wrote:
> > > >         if (up->port.sysrq) {
> > > >                 /* serial8250_handle_port() already took the lock */
> > > >                 locked = 0;
> > 
> > 
> > In this case it had interrupts off in the !PREEMPT_RT case, but your 
> > change leaves them on here.. _irqsave only runs in two of the three 
> > cases..
> 
> doh, right you are. I updated to the patch below.
> 
> 	Ingo


On closer inspection I still think this is wrong. (Although it looks
really nice..) find below speaking only in term of !PREEMPT_RT ,


> Index: linux/drivers/serial/8250.c
> ===================================================================
> --- linux.orig/drivers/serial/8250.c
> +++ linux/drivers/serial/8250.c
> @@ -2252,14 +2252,10 @@ serial8250_console_write(struct console 
>  
>  	touch_nmi_watchdog();
>  
> -	local_irq_save(flags);
> -	if (up->port.sysrq) {
> -		/* serial8250_handle_port() already took the lock */
> -		locked = 0;

in the old version interrupts are off, and stay off until the function
returns in all cases. Even "locked = 0" .

> -	} else if (oops_in_progress) {
> -		locked = spin_trylock(&up->port.lock);
> -	} else
> -		spin_lock(&up->port.lock);
> +	if (up->port.sysrq || oops_in_progress)
> +		locked = spin_trylock_irqsave(&up->port.lock, flags);

Now in the new version interrupts are only off if you _get the lock_.
Presumably the lock is taken in the calling function, but interrupts
aren't disabled. 

I'm assuming the code is disabling interrupts for a good reason, I don't
know enough about the code to say it isn't.

> +	else
> +		spin_lock_irqsave(&up->port.lock, flags);
>  
>  	/*
>  	 *	First save the IER then disable the interrupts
> @@ -2281,8 +2277,7 @@ serial8250_console_write(struct console 
>  	serial_out(up, UART_IER, ier);
>  
>  	if (locked)
> -		spin_unlock(&up->port.lock);
> -	local_irq_restore(flags);
> +		spin_unlock_irqrestore(&up->port.lock, flags);
>  }
>  
>  static int serial8250_console_setup(struct console *co, char *options)
> -
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