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

[Petr Mladek <pmladek@suse.com>]

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