From: Ingo Molnar <mingo@elte.hu>
Subject: detect atomic counter underflows

From: Ingo Molnar <mingo@elte.hu>

The patch below will detect atomic counter underflows.  This has been
test-driven in the -RT patchset for some time.  qdisc_destroy() triggered
it sometimes (in a seemingly nonfatal way, during device shutdown) - with
DaveM suggesting that it is most likely a bug in the networking code.  So
it would be nice to have this in -mm for some time to validate all atomic
counters on a broader base.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 arch/x86/include/asm/atomic_32.h |    5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/x86/include/asm/atomic_32.h
+++ b/arch/x86/include/asm/atomic_32.h
@@ -113,6 +113,11 @@ static inline int atomic_dec_and_test(at
 {
 	unsigned char c;
 
+	if (!atomic_read(v)) {
+		printk("BUG: atomic counter underflow at:\n");
+		dump_stack();
+	}
+
 	asm volatile(LOCK_PREFIX "decl %0; sete %1"
 		     : "+m" (v->counter), "=qm" (c)
 		     : : "memory");