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

Nikanth Karthikesan
2009-04-30T12:29:20Z

On Thursday 30 April 2009 17:51:59 Ingo Molnar wrote:
> * Nikanth Karthikesan <knikanth@novell.com> wrote:
> > On Thursday 30 April 2009 16:17:29 Ingo Molnar wrote:
> > > * Nikanth Karthikesan <knikanth@novell.com> wrote:
> > > > > As a generic debug helper this is not appropriate i think -
> > > > > counts can easily have a meaning when going negative as well.
> > > > > (we have no signed-atomic primitives)
> > > >
> > > > This doesn't warn when it becomes negative/positive from zero, but
> > > > only when it wraps around^W^Woverflows, trying to add past INT_MAX
> > > > or subtract from INT_MIN.
> > >
> > > ah! The small difference between INT_MAX and UINT_MAX.
> > >
> > > Sure, this debug check makes a lot of sense. -mm even had such
> > > stuff, many years ago? Never went upstream.
> > >
> > > > > > static inline void atomic_inc(atomic_t *v)
> > > > > > {
> > > > > > +#if defined(CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP)
> > > > > > + WARN_ON(atomic_add_unless(v, 1, INT_MAX) == 0);
> > > > > > +#else
> > > > > > asm volatile(LOCK_PREFIX "incl %0"
> > > > > >
> > > > > > : "+m" (v->counter));
> > > > > >
> > > > > > +#endif
> > > > > > }
> > > > >
> > > > > also looks a bit ugly - this ugly #ifdef would spread into every
> > > > > architecture.
> > > > >
> > > > > If we want to restrict atomic_t value ranges like that then the
> > > > > clean solution would be to add generic wrappers doing the debug
> > > > > (once, in generic code), and renaming the arch primitives to
> > > > > raw_atomic_inc() (etc), doing the lowlevel bits cleanly.
> > > >
> > > > Here is a patch which does it this way.
> > > >
> > > > 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.
> > > >
> > > > 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..6eda22b 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>
> > > > +
> > > > +#if defined(CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP)
> > >
> > > #ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP
>
> you missed this feedback :-)
>

Yes, sorry. Fixed Now. Thanks for the review. :-)

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..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
+

/*
* 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