linux-kernel - Re: [PATCH 2/2] printk: Provide a wake_up

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Date:	Thu, 14 Mar 2013 13:39:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	James Hogan <james.hogan@...tec.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] printk: Provide a wake_up_klogd() off-case

On Thu, 14 Mar 2013 15:26:29 +0100 Frederic Weisbecker <fweisbec@...il.com> wrote:

> wake_up_klogd() is useless when CONFIG_PRINTK=n because
> neither printk() nor printk_sched() are in use and there
> are actually no waiter on log_wait waitqueue. It should
> be a stub in this case for users like bust_spinlocks().
> 
> Otherwise this results in this warning when CONFIG_PRINTK=n
> and CONFIG_IRQ_WORK=n:
> 
> 	kernel/built-in.o In function `wake_up_klogd':
> 	(.text.wake_up_klogd+0xb4): undefined reference to `irq_work_queue'
> 
> To fix this, provide an off-case for wake_up_klogd() when
> CONFIG_PRINTK=n.
> 
> There is much more from console_unlock() and other console
> related code in printk.c that should be moved under
> CONFIG_PRINTK. But for now, focus on a minimal fix as we passed
> the merged window already.
> 
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -390,7 +390,6 @@ extern struct pid *session_of_pgrp(struct pid *pgrp);
>  unsigned long int_sqrt(unsigned long);
>  
>  extern void bust_spinlocks(int yes);
> -extern void wake_up_klogd(void);
>  extern int oops_in_progress;		/* If set, an oops, panic(), BUG() or die() is in progress */
>  extern int panic_timeout;
>  extern int panic_on_oops;
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 1249a54..822171f 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -134,6 +134,8 @@ extern int printk_delay_msec;
>  extern int dmesg_restrict;
>  extern int kptr_restrict;
>  
> +extern void wake_up_klogd(void);
> +

Given this, it would be prudent to include printk.h in
bust_spinlocks.c.  printk.h currently gets included by kernel.h, but
that doesn't actually seem necessary - nothing in kernel.h calls
printk().

(I pity anyone who tries to remove that include from kernel.h ;))

--- a/lib/bust_spinlocks.c~printk-provide-a-wake_up_klogd-off-case-fix
+++ a/lib/bust_spinlocks.c
@@ -8,6 +8,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/printk.h>
 #include <linux/spinlock.h>
 #include <linux/tty.h>
 #include <linux/wait.h>
@@ -28,5 +29,3 @@ void __attribute__((weak)) bust_spinlock
 			wake_up_klogd();
 	}
 }
-
-
_

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