From: Linus Torvalds <torvalds@osdl.org>
Verifies that all "atomic_dec_and_test()" users never see a negative value
(which would be bad).
I don't much expect this to trigger, but since a lot of data structures
(and the thread struct pages in particular) are protected with an atomic
counter, this just adds a bit of overflow protection. In particular, if
anything ends up releasing the thread structures too many times or
something like that, this should give a big warning.
arch/i386/lib/dec_and_lock.c | 2 +-
include/asm-i386/atomic.h | 13 ++++++++++---
2 files changed, 11 insertions(+), 4 deletions(-)
diff -puN arch/i386/lib/dec_and_lock.c~atomic_dec-debug arch/i386/lib/dec_and_lock.c
--- 25/arch/i386/lib/dec_and_lock.c~atomic_dec-debug 2003-11-02 10:09:11.000000000 -0800
+++ 25-akpm/arch/i386/lib/dec_and_lock.c 2003-11-02 10:09:11.000000000 -0800
@@ -19,7 +19,7 @@ repeat:
counter = atomic_read(atomic);
newcount = counter-1;
- if (!newcount)
+ if (newcount <= 0)
goto slow_path;
asm volatile("lock; cmpxchgl %1,%2"
diff -puN include/asm-i386/atomic.h~atomic_dec-debug include/asm-i386/atomic.h
--- 25/include/asm-i386/atomic.h~atomic_dec-debug 2003-11-02 10:09:11.000000000 -0800
+++ 25-akpm/include/asm-i386/atomic.h 2003-11-02 10:09:11.000000000 -0800
@@ -2,6 +2,8 @@
#define __ARCH_I386_ATOMIC__
#include <linux/config.h>
+#include <linux/kernel.h>
+#include <asm/bug.h>
/*
* Atomic operations that C can't guarantee us. Useful for
@@ -136,12 +138,17 @@ static __inline__ void atomic_dec(atomic
*/
static __inline__ int atomic_dec_and_test(atomic_t *v)
{
- unsigned char c;
+ static int count = 2;
+ unsigned char c, neg;
__asm__ __volatile__(
- LOCK "decl %0; sete %1"
- :"=m" (v->counter), "=qm" (c)
+ LOCK "decl %0; sete %1; sets %2"
+ :"=m" (v->counter), "=qm" (c), "=qm" (neg)
:"m" (v->counter) : "memory");
+ if (count && neg) {
+ count--;
+ WARN_ON(neg);
+ }
return c != 0;
}
_