Add the end-of-trace marker and the module list to WARN_ON()

Subject: Add the end-of-trace marker and the module list to WARN_ON()
From: Arjan van de Ven <[email protected]>
CC: Ingo Molnar <[email protected]>
CC: Andrew Morton <[email protected]>
CC: Olof Johansson <[email protected]>

Unlike oopses, WARN_ON() currently doesn't print the loaded modules list. This makes it harder to take action on certain bug reports. For example, recently there were a set of WARN_ON()s reported in the mac80211 stack, which were just signaling a driver bug. It takes then anther round trip to the bug reporter (if he responds at all) to find out which driver is at fault.

Another issue is that, unlike oopses, WARN_ON() doesn't currently printk the helpful "cut here" line, nor the "end of trace" marker. Now that WARN_ON() is out of line, the size increase due to this is minimal and it's worth adding.

---
 kernel/panic.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

Index: linux-2.6.24-rc6/kernel/panic.c
===================================================================
@@ -281,6 +281,13 @@ static int init_oops_id(void)
 }
 late_initcall(init_oops_id);
 
+static void print_oops_end_marker(void)
+{
+	init_oops_id();
+	printk(KERN_WARNING "---[ end trace %016llx ]---\n",
+		(unsigned long long)oops_id);
+}
+
 /*
  * Called when the architecture exits its oops handler, after printing
  * everything.

@@ -288,9 +295,7 @@ late_initcall(init_oops_id);
 void oops_exit(void)
 {
 	do_oops_enter_exit();
-	init_oops_id();
-	printk(KERN_WARNING "---[ end trace %016llx ]---\n",
-		(unsigned long long)oops_id);
+	print_oops_end_marker();
 }
 
 

@@ -298,11 +303,14 @@ void warn_on_slowpath(const char *file,
 {
 	char function[KSYM_SYMBOL_LEN];
 	unsigned long caller = (unsigned long) __builtin_return_address(0);
-
 	sprint_symbol(function, caller);
+
+	printk(KERN_WARNING "------------[ cut here ]------------\n");
 	printk(KERN_WARNING "WARNING: at %s:%d %s()\n", file,
 		line, function);
+	print_modules();
 	dump_stack();
+	print_oops_end_marker();
 }
 EXPORT_SYMBOL(warn_on_slowpath);
 

Subject: Add the end-of-trace marker and the module list to WARN_ON()
From: Arjan van de Ven <[email protected]>
CC: Ingo Molnar <[email protected]>
CC: Andrew Morton <[email protected]>
CC: Olof Johansson <[email protected]>

Unlike oopses, WARN_ON() currently does't print the loaded modules list. This makes it harder to take action on certain bug reports. For example, recently there were a set of WARN_ON()s reported in the mac80211 stack, which were just signalling a driver bug. It takes then anther round trip to the bug reporter (if he responds at all) to find out which driver is at fault.

Another issue is that, unlike oopses, WARN_ON() doesn't currently printk the helpful "cut here" line, nor the "end of trace" marker. Now that WARN_ON() is out of line, the size increase due to this is minimal and it's worth adding.

---
 kernel/panic.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

Index: linux-2.6.24-rc6/kernel/panic.c
===================================================================
@@ -281,6 +281,13 @@ static int init_oops_id(void)
 }
 late_initcall(init_oops_id);
 
+static void print_oops_end_marker(void)
+{
+	init_oops_id();
+	printk(KERN_WARNING "---[ end trace %016llx ]---\n",
+		(unsigned long long)oops_id);
+}
+
 /*
  * Called when the architecture exits its oops handler, after printing
  * everything.

@@ -288,9 +295,7 @@ late_initcall(init_oops_id);
 void oops_exit(void)
 {
 	do_oops_enter_exit();
-	init_oops_id();
-	printk(KERN_WARNING "---[ end trace %016llx ]---\n",
-		(unsigned long long)oops_id);
+	print_oops_end_marker();
 }
 
 

@@ -298,11 +303,14 @@ void warn_on_slowpath(const char *file, 
 {
 	char function[KSYM_SYMBOL_LEN];
 	unsigned long caller = (unsigned long) __builtin_return_address(0);
-
 	sprint_symbol(function, caller);
+
+	printk(KERN_WARNING "------------[ cut here ]------------\n");
 	printk(KERN_WARNING "WARNING: at %s:%d %s()\n", file,
 		line, function);
+	print_modules();
 	dump_stack();
+	print_oops_end_marker();
 }
 EXPORT_SYMBOL(warn_on_slowpath);
 

I have a preliminary patch for that already but I'm not happy with it yet; I need to redo it properly. It's neither depending nor incremental to these 2 patches though..

yeah, it was a sidenote - it should be separate to these two patches.

Ingo

it ends up printing the "end of trace" line as part of the oops_exit() call

as it should.. WARN_ON()'s are warnings so get KERN_WARNING ;-)

is this still the case after Olof's patch?

it's actually ok as is.. the core tracing code lives in panic.c; lib/bug.c is just a few helpers for BUG().. for WARN_ON() there is a lot less to help

Not on architectures that implement WARN_ON() with the help of lib/bug.c and an invalid opcode. Your out of line helper function doesn't get called there.

Instead the exception handler calls report_bug() which in turn prints the warning message, then calls show_regs() and returns BUG_TRAP_TYPE_WARN.

Afterwards the instruction pointer is incremented and execution continues.

This is true for powerpc, avr32, sh, parisc and s390.

Your out of line changes don't have any effect on these architectures. Dunno.. maybe you are aware of this. Just wanted to make sure we aren't talking of totally different things.

Not on all architectures.. see above :)

Yes, of course. Also I like the WARN_ON() with exception semantics much more since it gives the ability to print the register contents etc. So you get much more debug informations.

Not really... lib/bug.c handles both if the architecture wants it. It just needs to set BUGFLAG_WARNING in the bug table entries for warnings.