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SubjectRe: [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@am.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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