Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping around

[Nikanth Karthikesan <knikanth@novell.com>]

On Thursday 30 April 2009 19:07:57 Ingo Molnar wrote:
> * Nikanth Karthikesan <knikanth@novell.com> wrote:
> > > Then there could be a single, straightforward value check:
> > >
> > > static inline void atomic_inc(atomic_t *v)
> > > {
> > > 	debug_atomic_check_value(v);
> > > 	raw_atomic_inc(v);
> > > }
> > >
> > > Where debug_atomic_check_value() is just an atomic_read():
> > >
> > > static inline void debug_atomic_check_value(atomic_t *v)
> > > {
> > > 	WARN_ONCE(in_range(atomic_read(v), UINT_MAX/4, UINT_MAX/4*3),
> > > 		  KERN_ERR "atomic counter check failure!");
> > > }
> >
> > I do not understand, why UINT_MAX/4 to UINT_MAX/4*3?
> > Roughly,
> > UINT_MAX/4 = INT_MAX/2
> > UINT_MAX/4*3 = INT_MAX/2*3 which we will never reach with an int.
>
> i mean:
>
>  	WARN_ONCE(in_range((u32)atomic_read(v), UINT_MAX/4, UINT_MAX/4*3),
>  		  KERN_ERR "atomic counter check failure!");
>
> that's a single range check on an u32, selecting 'too large' and
> 'too small' s32 values.
>
> > > It's a constant check.
> > >
> > > If are overflowing on such a massive rate, it doesnt matter how
> > > early or late we check the value.
> >
> > UINT_MAX/4 early, might be too early. And if it doesn't matter how
> > early or late, why try to be over-cautious and produce false
> > warnings. ;-)
>
> UINT_MAX/4 is ~1 billion. If we reach a value of 1 billion we are
> leaking. Your check basically is a sharp test for the specific case
> of overflowing the boundary - but it makes the code slower (it uses
> more complex atomic ops) and uglifies it via #ifdefs as well.
>
> It doesnt matter whether we wrap over at around +2 billion into -2
> billion, or treat the whole above-1-billion and
> below-minus-1-billion range as invalid. (other than we'll catch bugs
> sooner via this method, and have faster and cleaner code)
>

Ah.. got it. But, range checking is not required as we are just verifying it
during increment and decrement, not atomic_add, atomic_sub etc... Should we
add debug checks to those operations as well? If we want to test those
operations as well, range check would be useful.

Here is a patch, without the overhead of a costly atomic operation which would
warn if it goes out of [INT_MIN/2 .. INT_MAX/2].

Thanks
Nikanth


Detect and warn on atomic_inc/atomic_dec overflow.

Add a debug option to detect and warn when the 32-bit atomic_t overflows
during atomic_inc and atomic_dec.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Acked-by: Ingo Molnar <mingo@elte.hu>

---


diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index df9e885..dd49be3 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -46,6 +46,7 @@ config X86
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_LZMA
+	select HAVE_ARCH_DEBUG_ATOMIC
 
 config ARCH_DEFCONFIG
 	string
diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h
index 85b46fb..c6a17bb 100644
--- a/arch/x86/include/asm/atomic_32.h
+++ b/arch/x86/include/asm/atomic_32.h
@@ -78,24 +78,24 @@ static inline int atomic_sub_and_test(int i, atomic_t *v)
 }
 
 /**
- * atomic_inc - increment atomic variable
+ * raw_atomic_inc - increment atomic variable
  * @v: pointer of type atomic_t
  *
  * Atomically increments @v by 1.
  */
-static inline void atomic_inc(atomic_t *v)
+static inline void raw_atomic_inc(atomic_t *v)
 {
 	asm volatile(LOCK_PREFIX "incl %0"
 		     : "+m" (v->counter));
 }
 
 /**
- * atomic_dec - decrement atomic variable
+ * raw_atomic_dec - decrement atomic variable
  * @v: pointer of type atomic_t
  *
  * Atomically decrements @v by 1.
  */
-static inline void atomic_dec(atomic_t *v)
+static inline void raw_atomic_dec(atomic_t *v)
 {
 	asm volatile(LOCK_PREFIX "decl %0"
 		     : "+m" (v->counter));
diff --git a/arch/x86/include/asm/atomic_64.h b/arch/x86/include/asm/atomic_64.h
index 8c21731..1183b85 100644
--- a/arch/x86/include/asm/atomic_64.h
+++ b/arch/x86/include/asm/atomic_64.h
@@ -77,12 +77,12 @@ static inline int atomic_sub_and_test(int i, atomic_t *v)
 }
 
 /**
- * atomic_inc - increment atomic variable
+ * raw_atomic_inc - increment atomic variable
  * @v: pointer of type atomic_t
  *
  * Atomically increments @v by 1.
  */
-static inline void atomic_inc(atomic_t *v)
+static inline void raw_atomic_inc(atomic_t *v)
 {
 	asm volatile(LOCK_PREFIX "incl %0"
 		     : "=m" (v->counter)
@@ -90,12 +90,12 @@ static inline void atomic_inc(atomic_t *v)
 }
 
 /**
- * atomic_dec - decrement atomic variable
+ * raw_atomic_dec - decrement atomic variable
  * @v: pointer of type atomic_t
  *
  * Atomically decrements @v by 1.
  */
-static inline void atomic_dec(atomic_t *v)
+static inline void raw_atomic_dec(atomic_t *v)
 {
 	asm volatile(LOCK_PREFIX "decl %0"
 		     : "=m" (v->counter)
diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h
index 7abdaa9..007931a 100644
--- a/include/asm-generic/atomic.h
+++ b/include/asm-generic/atomic.h
@@ -4,15 +4,51 @@
  * Copyright (C) 2005 Silicon Graphics, Inc.
  *	Christoph Lameter
  *
- * Allows to provide arch independent atomic definitions without the need to
- * edit all arch specific atomic.h files.
  */
 
+#include <linux/kernel.h>
 #include <asm/types.h>
+#include <asm/bug.h>
+
+
+/**
+ * atomic_inc - increment atomic variable
+ * @v: pointer of type atomic_t
+ *
+ * Atomically increments @v by 1.
+ */
+static inline void atomic_inc(atomic_t *v)
+{
+#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
+	WARN_ONCE((atomic_read(v) > (INT_MAX / 2)),
+		KERN_ERR "atomic counter check failure!");
+#endif
+	raw_atomic_inc(v);
+}
+
+/**
+ * atomic_dec - decrement atomic variable
+ * @v: pointer of type atomic_t
+ *
+ * Atomically decrements @v by 1.
+ */
+static inline void atomic_dec(atomic_t *v)
+{
+#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
+	WARN_ONCE((atomic_read(v) < (INT_MIN / 2)),
+		KERN_ERR "atomic counter check failure!");
+#endif
+	raw_atomic_dec(v);
+
+}
+
 
 /*
  * Suppport for atomic_long_t
  *
+ * Allows to provide arch independent atomic definitions without the need to
+ * edit all arch specific atomic.h files.
+ *
  * Casts for parameters are avoided for existing atomic functions in order to
  * avoid issues with cast-as-lval under gcc 4.x and other limitations that the
  * macros of a platform may have.
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 812c282..773c1a4 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -17,6 +17,17 @@ config ENABLE_WARN_DEPRECATED
 	  Disable this to suppress the "warning: 'foo' is deprecated
 	  (declared at kernel/power/somefile.c:1234)" messages.
 
+config HAVE_ARCH_DEBUG_ATOMIC
+	bool
+
+config ENABLE_WARN_ATOMIC_INC_WRAP
+	bool "Enable warning on atomic_inc()/atomic_dec() wrap"
+	depends on HAVE_ARCH_DEBUG_ATOMIC
+	default y
+	help
+	  Enable printing a warning when atomic_inc() or atomic_dec()
+	  operation wraps around the 32-bit value.
+
 config ENABLE_MUST_CHECK
 	bool "Enable __must_check logic"
 	default y