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

[Nikanth Karthikesan]

On Thursday 30 April 2009 18:20:04 Ingo Molnar wrote:
> * Nikanth Karthikesan <knikanth@xxxxxxxxxx> wrote:
> > diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h
> > index 7abdaa9..edf5619 100644
> > --- a/include/asm-generic/atomic.h
> > +++ b/include/asm-generic/atomic.h
> > @@ -4,15 +4,71 @@
> >   * 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>
> > +
> > +#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
> > +
> > +/**
> > + * atomic_inc - increment atomic variable
> > + * @v: pointer of type atomic_t
> > + *
> > + * Atomically increments @v by 1.
> > + * Prints a warning if it wraps around.
> > + */
> > +static inline void atomic_inc(atomic_t *v)
> > +{
> > +	WARN_ON(atomic_add_unless(v, 1, INT_MAX) == 0);
> > +}
> > +
> > +/**
> > + * atomic_dec - decrement atomic variable
> > + * @v: pointer of type atomic_t
> > + *
> > + * Atomically decrements @v by 1.
> > + * Prints a warning if it wraps around.
> > + */
> > +static inline void atomic_dec(atomic_t *v)
> > +{
> > +	WARN_ON(atomic_add_unless(v, -1, INT_MIN) == 0);
> > +}
> > +
> > +#else
> > +
> > +/**
> > + * atomic_inc - increment atomic variable
> > + * @v: pointer of type atomic_t
> > + *
> > + * Atomically increments @v by 1.
> > + */
> > +static inline void atomic_inc(atomic_t *v)
> > +{
> > +	raw_atomic_inc(v);
> > +}
> > +
> > +/**
> > + * atomic_dec - increment atomic variable
> > + * @v: pointer of type atomic_t
> > + *
> > + * Atomically decrements @v by 1.
> > + */
> > +static inline void atomic_dec(atomic_t *v)
> > +{
> > +	raw_atomic_dec(v);
> > +}
> > +
> > +#endif
>
> Here i think it makes sense to have a single definition:
>
> static inline void atomic_inc(atomic_t *v)
> {
> #ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
> 	WARN_ON(atomic_add_unless(v, 1, INT_MAX) == 0);
> #else
> 	raw_atomic_inc(v);
> #endif
> }
>

Ok, done.

> Or, better yet, i'd suggest to define an 'invalid value' range for
> atomic integer types - not restricted to the single value of
> INT_MAX, but in the range of: [ UINT_MAX/4 ... INT_MAX/4*3 ].
>

atomic_t is defined an int, why use UINT_MAX?

> 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.

> 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. ;-)

> Agreed?
>

Hmm.. partly yes, but not the range. Could the range be made as INT_MAX/4*3 to
INT_MAX? But even then, this might trigger false-positives, when atomic_t is
used in places where it was meant to exercise the full int range with the
assumption it would never go above INT_MAX.

If you still think that, it would be better to catch it early, I would send
you another patch with the range checking incorporated.

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@xxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>

---

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..ca47932 100644
--- a/include/asm-generic/atomic.h
+++ b/include/asm-generic/atomic.h
@@ -4,15 +4,52 @@
  * 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_ON(atomic_add_unless(v, 1, INT_MAX) == 0);
+#else
+	raw_atomic_inc(v);
+#endif
+}
+
+/**
+ * atomic_dec - decrement atomic variable
+ * @v: pointer of type atomic_t
+ *
+ * Atomically decrements @v by 1.
+ * Prints a warning if it wraps around.
+ */
+static inline void atomic_dec(atomic_t *v)
+{
+#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
+	WARN_ON(atomic_add_unless(v, -1, INT_MIN) == 0);
+#else
+	raw_atomic_dec(v);
+#endif
+
+}
+
 
 /*
  * 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

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