Subject: Re: [PATCH v3 1/7] dump_stack: Support adding to the dump stack arch description

From: Andrea Parri <[email-masked]>
Date: Mon, 11 Feb 2019 13:50:35 +0100

Hi Michael,

On Thu, Feb 07, 2019 at 11:46:29PM +1100, Michael Ellerman wrote:
> Arch code can set a "dump stack arch description string" which is
> displayed with oops output to describe the hardware platform.
> 
> It is useful to initialise this as early as possible, so that an early
> oops will have the hardware description.
> 
> However in practice we discover the hardware platform in stages, so it
> would be useful to be able to incrementally fill in the hardware
> description as we discover it.
> 
> This patch adds that ability, by creating dump_stack_add_arch_desc().
> 
> If there is no existing string it behaves exactly like
> dump_stack_set_arch_desc(). However if there is an existing string it
> appends to it, with a leading space.
> 
> This makes it easy to call it multiple times from different parts of the
> code and get a reasonable looking result.
> 
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  include/linux/printk.h |  5 ++++
>  lib/dump_stack.c       | 58 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 63 insertions(+)
> 
> v3: No change, just widened Cc list.
> 
> v2: Add a smp_wmb() and comment.
> 
> v1 is here for reference https://lore.kernel.org/lkml/[email-masked]/
> 
> I'll take this series via the powerpc tree if no one minds?
> 
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 77740a506ebb..d5fb4f960271 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -198,6 +198,7 @@ u32 log_buf_len_get(void);
>  void log_buf_vmcoreinfo_setup(void);
>  void __init setup_log_buf(int early);
>  __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
> +__printf(1, 2) void dump_stack_add_arch_desc(const char *fmt, ...);
>  void dump_stack_print_info(const char *log_lvl);
>  void show_regs_print_info(const char *log_lvl);
>  extern asmlinkage void dump_stack(void) __cold;
> @@ -256,6 +257,10 @@ static inline __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...)
>  {
>  }
>  
> +static inline __printf(1, 2) void dump_stack_add_arch_desc(const char *fmt, ...)
> +{
> +}
> +
>  static inline void dump_stack_print_info(const char *log_lvl)
>  {
>  }
> diff --git a/lib/dump_stack.c b/lib/dump_stack.c
> index 5cff72f18c4a..69b710ff92b5 100644
> --- a/lib/dump_stack.c
> +++ b/lib/dump_stack.c
> @@ -35,6 +35,64 @@ void __init dump_stack_set_arch_desc(const char *fmt, ...)
>  	va_end(args);
>  }
>  
> +/**
> + * dump_stack_add_arch_desc - add arch-specific info to show with task dumps
> + * @fmt: printf-style format string
> + * @...: arguments for the format string
> + *
> + * See dump_stack_set_arch_desc() for why you'd want to use this.
> + *
> + * This version adds to any existing string already created with either
> + * dump_stack_set_arch_desc() or dump_stack_add_arch_desc(). If there is an
> + * existing string a space will be prepended to the passed string.
> + */
> +void __init dump_stack_add_arch_desc(const char *fmt, ...)
> +{
> +	va_list args;
> +	int pos, len;
> +	char *p;
> +
> +	/*
> +	 * If there's an existing string we snprintf() past the end of it, and
> +	 * then turn the terminating NULL of the existing string into a space
> +	 * to create one string separated by a space.
> +	 *
> +	 * If there's no existing string we just snprintf() to the buffer, like
> +	 * dump_stack_set_arch_desc(), but without calling it because we'd need
> +	 * a varargs version.
> +	 */
> +	len = strnlen(dump_stack_arch_desc_str, sizeof(dump_stack_arch_desc_str));
> +	pos = len;
> +
> +	if (len)
> +		pos++;
> +
> +	if (pos >= sizeof(dump_stack_arch_desc_str))
> +		return; /* Ran out of space */
> +
> +	p = &dump_stack_arch_desc_str[pos];
> +
> +	va_start(args, fmt);
> +	vsnprintf(p, sizeof(dump_stack_arch_desc_str) - pos, fmt, args);
> +	va_end(args);
> +
> +	if (len) {
> +		/*
> +		 * Order the stores above in vsnprintf() vs the store of the
> +		 * space below which joins the two strings. Note this doesn't
> +		 * make the code truly race free because there is no barrier on
> +		 * the read side. ie. Another CPU might load the uninitialised
> +		 * tail of the buffer first and then the space below (rather
> +		 * than the NULL that was there previously), and so print the
> +		 * uninitialised tail. But the whole string lives in BSS so in
> +		 * practice it should just see NULLs.

The comment doesn't say _why_ we need to order these stores: IOW, what
will or can go wrong without this order?  This isn't clear to me.

Another good practice when adding smp_*-constructs (as discussed, e.g.,
at KS'18) is to indicate the matching construct/synch. mechanism.

  Andrea

> +		 */
> +		smp_wmb();
> +
> +		dump_stack_arch_desc_str[len] = ' ';
> +	}
> +}
> +
>  /**
>   * dump_stack_print_info - print generic debug info for dump_stack()
>   * @log_lvl: log level
> -- 
> 2.20.1
>