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

Andrea Parri
2019-02-19T23:39:25Z

On Mon, Feb 11, 2019 at 03:38:59PM +0100, Petr Mladek wrote:
> On Mon 2019-02-11 13:50:35, Andrea Parri wrote:
> > 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.
>
> Yes, one barrier without a counter-part is suspicious.

As is this silence...,

Michael, what happened to this patch? did you submit a new version?

>
> If the parallel access is really needed then we could define the
> current length as atomic_t and use:
>
> + atomic_cmpxchg() to reserve the space for the string
> + %*s to limit the printed length
>
> In the worst case, we would print an incomplete string.
> See below for a sample code.

Seems worth exploring, IMO; but I'd like to first hear _clear about
the _intended semantics (before digging into alternatives)...

+rostedt, who first raised the question about "parallel accesses"

http://lkml.kernel.org/r/20190208185515.r6vkrezbd3odhpxt@home.goodmis.org

Andrea

>
>
> BTW: There are very few users of dump_stack_set_arch_desc().
> I would use dump_stack_add_arch_desc() everywhere to keep
> it simple and have a reasonable semantic.
>
>
> This is what I mean (only compile tested):
>
> diff --git a/lib/dump_stack.c b/lib/dump_stack.c
> index 5cff72f18c4a..311dd20cc6a7 100644
> --- a/lib/dump_stack.c
> +++ b/lib/dump_stack.c
> @@ -14,9 +14,10 @@
> #include <linux/utsname.h>
>
> static char dump_stack_arch_desc_str[128];
> +static atomic_t arch_desc_str_len;
>
> /**
> - * dump_stack_set_arch_desc - set arch-specific str to show with task dumps
> + * dump_stack_set_arch_desc - add arch-specific str to show with task dumps
> * @fmt: printf-style format string
> * @...: arguments for the format string
> *
> @@ -25,13 +26,32 @@ static char dump_stack_arch_desc_str[128];
> * arch wants to make use of such an ID string, it should initialize this
> * as soon as possible during boot.
> */
> -void __init dump_stack_set_arch_desc(const char *fmt, ...)
> +void __init dump_stack_add_arch_desc(const char *fmt, ...)
> {
> - va_list args;
> + va_list args, args2;
> + int len, cur_len, old_len;
>
> va_start(args, fmt);
> - vsnprintf(dump_stack_arch_desc_str, sizeof(dump_stack_arch_desc_str),
> +
> + va_copy(args2, args);
> + len = vsnprintf(NULL, sizeof(dump_stack_arch_desc_str),
> + fmt, args2);
> + va_end(args2);
> +
> +try_again:
> + cur_len = atomic_read(&arch_desc_str_len);
> + if (cur_len + len > sizeof(dump_stack_arch_desc_str))
> + goto out;
> +
> + old_len = atomic_cmpxchg(&arch_desc_str_len,
> + cur_len, cur_len + len);
> + if (old_len != cur_len)
> + goto try_again;
> +
> + vsnprintf(dump_stack_arch_desc_str + old_len,
> + sizeof(dump_stack_arch_desc_str) - old_len,
> fmt, args);
> +out:
> va_end(args);
> }
>
> @@ -44,6 +64,8 @@ void __init dump_stack_set_arch_desc(const char *fmt, ...)
> */
> void dump_stack_print_info(const char *log_lvl)
> {
> + int len;
> +
> printk("%sCPU: %d PID: %d Comm: %.20s %s%s %s %.*s\n",
> log_lvl, raw_smp_processor_id(), current->pid, current->comm,
> kexec_crash_loaded() ? "Kdump: loaded " : "",
> @@ -52,9 +74,11 @@ void dump_stack_print_info(const char *log_lvl)
> (int)strcspn(init_utsname()->version, " "),
> init_utsname()->version);
>
> - if (dump_stack_arch_desc_str[0] != '\0')
> - printk("%sHardware name: %s\n",
> - log_lvl, dump_stack_arch_desc_str);
> + len = atomic_read(&arch_desc_str_len);
> + if (len) {
> + printk("%sHardware name: %*s\n",
> + log_lvl, len, dump_stack_arch_desc_str);
> + }
>
> print_worker_info(log_lvl, current);
> }
>
> Best Regards,
> Petr