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

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?

Cc-ing Steven

  https://lore.kernel.org/lkml/20190207124635.3885-1-mpe@ellerman.id.au/T/#u

On (02/07/19 23:46), 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>

You probably can have a __init buffer somewhere in ppc code, append
data to it, step by step, and call dump_stack_set_arch_desc() all
the time.

But no real objections; dump_stack_add_arch_desc() can do.

FWIW,
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

	-ss

On Thu, Feb 07, 2019 at 11:46:29PM +1100, Michael Ellerman wrote:
> 
> 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.
> +		 */
> +		smp_wmb();

This shows me that this can be called at a time when more than one CPU is
active. What happens if we have two CPUs calling dump_stack_add_arch_desc() at
the same time? Can't that corrupt the dump_stack_arch_desc_str?

-- Steve

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

On (02/08/19 13:55), Steven Rostedt wrote:
[..]
> > +	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.
> > +		 */
> > +		smp_wmb();
> 
> This shows me that this can be called at a time when more than one CPU is
> active. What happens if we have two CPUs calling dump_stack_add_arch_desc() at
> the same time? Can't that corrupt the dump_stack_arch_desc_str?

Can overwrite part of it, I guess (but it seems that Michael
is OK with this). The string is still NULL terminated.

The worst case scenario I can think of is not the one when
two CPUs call dump_stack_add_arch_desc(), but when CPUA calls
dump_stack_add_arch_desc() to append some data and at the
same time CPUB calls dump_stack_set_arch_desc() and simply
overwrites dump_stack_arch_desc_str. Not sure if this is
critical (or possible).

	-ss

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
>

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

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

Andrea Parri <andrea.parri@amarulasolutions.com> writes:
> On Mon, Feb 11, 2019 at 03:38:59PM +0100, Petr Mladek wrote:
>> On Mon 2019-02-11 13:50:35, Andrea Parri wrote:
>> > 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?

No, I'm just busy, it's the merge window next week :)

I thought the comment was pretty clear, if the stores are observed out
of order we might print the uninitialised tail.

And the barrier on the read side would need to be in printk somewhere,
which is obviously unpleasant.

>> 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)...

It is not my intention to support concurrent updates of the string. The
idea is you setup the string early in boot.

The concern with a concurrent reader is simply that the string is dumped
in the panic path, and you never really know when you're going to panic.
Even if you only write to the string before doing SMP bringup you might
still have another CPU go rogue and panic before then.

But I probably should have just not added the barrier, it's over
paranoid and will almost certainly never matter in practice.

cheers

> >> > > +		 * 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?
> 
> No, I'm just busy, it's the merge window next week :)

Got it.


> 
> I thought the comment was pretty clear, if the stores are observed out
> of order we might print the uninitialised tail.
> 
> And the barrier on the read side would need to be in printk somewhere,
> which is obviously unpleasant.

Indeed.


> 
> >> 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)...
> 
> It is not my intention to support concurrent updates of the string. The
> idea is you setup the string early in boot.

Understood, thanks for the clarification.


> 
> The concern with a concurrent reader is simply that the string is dumped
> in the panic path, and you never really know when you're going to panic.
> Even if you only write to the string before doing SMP bringup you might
> still have another CPU go rogue and panic before then.
> 
> But I probably should have just not added the barrier, it's over
> paranoid and will almost certainly never matter in practice.

Oh, well, I can only echo you: if you don't care about the stores being
_observed_ out of order, you could simply remove the barrier; if you do
care, then you need "more paranoid" on the readers side.  ;-)

  Andrea


> 
> cheers

On Wed 2019-02-20 14:44:33, Andrea Parri wrote:
> > >> > > +		 * 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.
> > >> > 
> > It is not my intention to support concurrent updates of the string. The
> > idea is you setup the string early in boot.
> 
> Understood, thanks for the clarification.
> > 
> > The concern with a concurrent reader is simply that the string is dumped
> > in the panic path, and you never really know when you're going to panic.
> > Even if you only write to the string before doing SMP bringup you might
> > still have another CPU go rogue and panic before then.
> > 
> > But I probably should have just not added the barrier, it's over
> > paranoid and will almost certainly never matter in practice.
> 
> Oh, well, I can only echo you: if you don't care about the stores being
> _observed_ out of order, you could simply remove the barrier; if you do
> care, then you need "more paranoid" on the readers side.  ;-)

Hmm, the barrier might be fine and actually useful. The
purpose is to make sure that the later '\0' is written before
the existing one is replaced by ' '.

The reader does not need the barrier as long as it reads the string
sequentially. I would expect that it is always the case. But who
knows with all the speculation-related CPU bugs around.

In each case, any race could never crash the kernel.
The dump_stack_arch_desc_str is zeroed out of box and
the very last '\0' is never rewritten.

Best Regards,
Petr