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Message-Id: <200904301939.51022.knikanth@novell.com>
Date: Thu, 30 Apr 2009 19:39:50 +0530
From: Nikanth Karthikesan <knikanth@...ell.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Jens Axboe <jens.axboe@...cle.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping around
On Thursday 30 April 2009 19:35:59 Ingo Molnar wrote:
> * Nikanth Karthikesan <knikanth@...ell.com> wrote:
> > On Thursday 30 April 2009 19:07:57 Ingo Molnar wrote:
> > > * Nikanth Karthikesan <knikanth@...ell.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.
>
> Good point! Indeed the checks can be even simpler that way - a
> single test.
>
> > 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].
> >
> > +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!");
>
> here the message can be more specific i think:
>
> KERN_ERR "atomic inc overflow!");
>
> > +#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
>
> and here:
>
> KERN_ERR "atomic inc underflow!");
>
> other than these two small details this is looking really nice now.
>
Done.
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@...e.de>
Acked-by: Ingo Molnar <mingo@...e.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..e0ffa32 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 inc overflow!");
+#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 dec underflow!");
+#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
--
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