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

Andrea Parri
2019-02-11T12:50:48Z

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 <mpe@ellerman.id.au>
> ---
> 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/1430824337-15339-1-git-send-email-mpe@ellerman.id.au/
>
> 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
>