linux-kernel - Re: [RFC PATCH 14/17] printk: Add per_cpu printk func to allow printk to be diverted

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Date:	Mon, 18 May 2015 15:56:03 +0200
From:	Petr Mladek <pmladek@...e.cz>
To:	Wang Long <long.wanglong@...wei.com>
Cc:	rostedt@...dmis.org, jkosina@...e.cz, gregkh@...uxfoundation.org,
	stable@...r.kernel.org, wanglong@...qinren.net,
	peifeiyue@...wei.com, linux-kernel@...r.kernel.org,
	morgan.wang@...wei.com, paulmck@...ux.vnet.ibm.com,
	dzickus@...hat.com, x86@...nel.org, sasha.levin@...cle.com
Subject: Re: [RFC PATCH 14/17] printk: Add per_cpu printk func to allow
 printk to be diverted

On Thu 2015-05-14 11:35:01, Wang Long wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> 
> commit afdc34a3d3b823a12a93b822ee1efb566f884032 upstream.
> 
> Being able to divert printk to call another function besides the normal
> logging is useful for such things like NMI handling. If some functions
> are to be called from NMI that does printk() it is possible to lock up
> the box if the nmi handler triggers when another printk is happening.
> 
> One example of this use is to perform a stack trace on all CPUs via NMI.
> But if the NMI is to do the printk() it can cause the system to lock up.
> By allowing the printk to be diverted to another function that can safely
> record the printk output and then print it when it in a safe context
> then NMIs will be safe to call these functions like show_regs().
> 
> Link: http://lkml.kernel.org/p/20140619213952.209176403@goodmis.org
> 
> Tested-by: Jiri Kosina <jkosina@...e.cz>
> Acked-by: Jiri Kosina <jkosina@...e.cz>
> Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Reviewed-by: Petr Mladek <pmladek@...e.cz>
> [wanglong: backport to 3.10 stable
> - adjust context
> ]
> Signed-off-by: Wang Long <long.wanglong@...wei.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  include/linux/percpu.h |  3 +++
>  include/linux/printk.h |  2 ++
>  kernel/printk.c        | 38 +++++++++++++++++++++++++++++---------
>  3 files changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index cc88172..bb1d29c 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -755,4 +755,7 @@ do {									\
>  	__pcpu_double_call_return_bool(__this_cpu_cmpxchg_double_, (pcp1), (pcp2), (oval1), (oval2), (nval1), (nval2))
>  #endif
>  
> +/* To avoid include hell, as printk can not declare this, we declare it here */
> +DECLARE_PER_CPU(printk_func_t, printk_func);
> +
>  #endif /* __LINUX_PERCPU_H */
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 708b8a8..ebc7f3e 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -144,6 +144,8 @@ extern int kptr_restrict;
>  
>  extern void wake_up_klogd(void);
>  
> +typedef int(*printk_func_t)(const char *fmt, va_list args);
> +
>  void log_buf_kexec_setup(void);
>  void __init setup_log_buf(int early);
>  void dump_stack_set_arch_desc(const char *fmt, ...);
> diff --git a/kernel/printk.c b/kernel/printk.c
> index fd0154a..d3d929d 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1657,6 +1657,30 @@ asmlinkage int printk_emit(int facility, int level,
>  }
>  EXPORT_SYMBOL(printk_emit);
>  
> +int vprintk_default(const char *fmt, va_list args)
> +{
> +       int r;
> +
> +#ifdef CONFIG_KGDB_KDB
> +       if (unlikely(kdb_trap_printk)) {
> +               r = vkdb_printf(fmt, args);
> +               return r;
> +       }
> +#endif
> +       r = vprintk_emit(0, -1, NULL, 0, fmt, args);
> +
> +       return r;

There is broken indentation in this hunk.

The rest of the patch looks fine.

Best Regards,
Petr
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