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

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Date:	Wed, 16 May 2012 18:09:36 -0700
From:	Frank Rowand <frank.rowand@...sony.com>
To:	"Rowand, Frank" <Frank_Rowand@...yusa.com>
CC:	"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: [PATCH RT 2/2] fix printk flush of messages


Updates console-make-rt-friendly.patch

#ifdef CONFIG_PREEMPT_RT_FULL, printk() output is never flushed by
printk() because:

   # 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()
         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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