linux-kernel - Re: [PATCH RT 2/2] fix printk flush of messages

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Message-ID: <4FBAA150.6050308@caviumnetworks.com>
Date:	Mon, 21 May 2012 13:10:56 -0700
From:	Venkat Subbiah <vsubbiah@...iumnetworks.com>
To:	"frank.rowand@...sony.com" <frank.rowand@...sony.com>
CC:	"Rowand, Frank" <Frank_Rowand@...yusa.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH RT 2/2] fix printk flush of messages

On 05/16/2012 06:09 PM, Frank Rowand wrote:
> Updates console-make-rt-friendly.patch
>
> #ifdef CONFIG_PREEMPT_RT_FULL, printk() output is never flushed by
> printk() because:
So this is an issue for printk() itself and is not just for early_printk()?


    # some liberties taken in this pseudo-code to make it easier to follow
    printk()
       vprintk()
          raw_spin_lock(&logbuf_lock)
             # increment preempt_count():
             preempt_disable()
       result = console_trylock_for_printk()

As I read it console_trylock_for_printk() is called from printk() but in 
code it is called from vprintk()
>           retval = 0
>           # lock will always be false, because preempt_count() will be>= 1
>           lock = ...&&  !preempt_count()
>           if (lock)
>              retval = 1
>           return retval
>        # result will always be false since lock will always be false
>        if (result)
>           console_unlock()
>              # this is where the printk() output would be flushed
>
>
> On system boot some printk() output is flushed because register_console()
> and tty_open() call console_unlock().
>
>
> This change also fixes the problem that was previously fixed by
> preempt-rt-allow-immediate-magic-sysrq-output-for-preempt_rt_full.patch
>
> Signed-off-by: Frank Rowand<frank.rowand@...sony.com>
>
> ---
>   kernel/printk.c |    2 	1 +	1 -	0 !
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: b/kernel/printk.c
> ===================================================================
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -847,7 +847,7 @@ static int console_trylock_for_printk(un
>   	int retval = 0, wake = 0;
>   #ifdef CONFIG_PREEMPT_RT_FULL
>   	int lock = !early_boot_irqs_disabled&&  !irqs_disabled_flags(flags)&&
> -		!preempt_count();
> +		(preempt_count()<= 1);
>   #else
>   	int lock = 1;
>   #endif
>
> --
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