Re: [RFC][PATCH 2/3] printk: Add per_cpu printk func to allow printk to be diverted

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jun 19, 2014 at 05:33:31PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> 
> 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().
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
>  include/linux/percpu.h |  3 +++
>  include/linux/printk.h |  2 ++
>  kernel/printk/printk.c | 38 +++++++++++++++++++++++++++++---------
>  3 files changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index 8419053d0f2e..9997c92ce3bd 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -802,4 +802,7 @@ do { __this_cpu_preempt_check("or");					\
>  	(__this_cpu_preempt_check("cmpxchg_double"),__pcpu_double_call_return_bool(raw_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 319ff7e53efb..e26310b2d2fd 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -159,6 +159,8 @@ 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/printk.c b/kernel/printk/printk.c
> index ea2d5f6962ed..d019ccd10d4c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1764,6 +1764,30 @@ 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;
> +}
> +EXPORT_SYMBOL_GPL(vprintk_default);
> +
> +/*
> + * This allows printk to be diverted to another function per cpu.
> + * This is useful for calling printk functions from within NMI
> + * without worrying about race conditions that can lock up the
> + * box.
> + */
> +DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
> +	
>  /**
>   * printk - print a kernel message
>   * @fmt: format string
> @@ -1787,19 +1811,15 @@ EXPORT_SYMBOL(printk_emit);
>   */
>  asmlinkage __visible int printk(const char *fmt, ...)
>  {
> +	printk_func_t vprintk_func;
>  	va_list args;
>  	int r;
>  
> -#ifdef CONFIG_KGDB_KDB
> -	if (unlikely(kdb_trap_printk)) {
> -		va_start(args, fmt);
> -		r = vkdb_printf(fmt, args);
> -		va_end(args);
> -		return r;
> -	}
> -#endif
>  	va_start(args, fmt);
> -	r = vprintk_emit(0, -1, NULL, 0, fmt, args);
> +	preempt_disable();
> +	vprintk_func = this_cpu_read(printk_func);
> +	r = vprintk_func(fmt, args);
> +	preempt_enable();
>  	va_end(args);
>  
>  	return r;
> -- 
> 2.0.0.rc2
> 
> 
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