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

Petr Mladek
2019-02-11T14:38:59Z

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.

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.

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