[patch 2.6.28-rc6] when to BUG(), and when to not BUG()

[David Brownell <david-b@pacbell.net>]

From: David Brownell <dbrownell@users.sourceforge.net>

Provide some basic advice about when to use BUG()/BUG_ON():
never, unless there's really no better option.

This matches my understanding of the standard policy ... which
seems not to be written down so far, outside of LKML messages
that I haven't bookmarked.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
---
This arguably belongs in patch checklists too... but I figure we
should start by flushing out any contrary opinions.

 include/asm-generic/bug.h |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -20,6 +20,17 @@ struct bug_entry {
 #define BUGFLAG_WARNING	(1<<0)
 #endif	/* CONFIG_GENERIC_BUG */
 
+/*
+ * Don't use BUG() or BUG_ON() unless there's really no way out; one
+ * example might be detecting data structure corruption in the middle
+ * of an operation that can't be backed out of.  If the (sub)system
+ * can somehow continue operating, perhaps with reduced functionality,
+ * it's probably not BUG-worthy.
+ *
+ * If you're tempted to BUG(), think again:  is completely giving up
+ * really the *only* solution?  There are usually better options, where
+ * users don't need to reboot ASAP and can mostly shut down cleanly.
+ */
 #ifndef HAVE_ARCH_BUG
 #define BUG() do { \
 	printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
@@ -31,6 +42,12 @@ struct bug_entry {
 #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)
 #endif
 
+/*
+ * WARN(), WARN_ON(), WARN_ON_ONCE, and so on can be used to report
+ * significant issues that need prompt attention if they should ever
+ * appear at runtime.  Use the versions with printk format strings
+ * to provide better diagnostics.
+ */
 #ifndef __WARN
 #ifndef __ASSEMBLY__
 extern void warn_on_slowpath(const char *file, const int line);