linux-kernel - Re: [patch v3 2/3] percpu_counter: fix code for 32bit systems for UP

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Date:	Mon, 18 Apr 2011 15:41:32 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tj@...nel.org" <tj@...nel.org>, "cl@...ux.com" <cl@...ux.com>
Subject: Re: [patch v3 2/3] percpu_counter: fix code for 32bit systems for
 UP

On Thu, 2011-04-14 at 10:54 +0800, Eric Dumazet wrote:
> Le jeudi 14 avril 2011 à 10:44 +0800, Shaohua Li a écrit :
> > that's true. I just want the code looks consistent and this doesn't
> > really obfuscate the code to me.
> 
> Doing so gives a false sense of security to user.
> 
> percpu_counter_init() is unsafe by nature.
I really don't think this matters, but since you insist, here is the
updated the patch.

Subject: percpu_counter: fix code for 32bit systems for UP

percpu_counter.counter is a 's64'. Accessing it in 32-bit system is racing.
we need some locking to protect it otherwise some very wrong value could be
accessed.

Signed-off-by: Shaohua Li <shaohua.li@...el.com>
---
 include/linux/percpu_counter.h |   31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

Index: linux/include/linux/percpu_counter.h
===================================================================
--- linux.orig/include/linux/percpu_counter.h	2011-04-18 15:36:31.000000000 +0800
+++ linux/include/linux/percpu_counter.h	2011-04-18 15:37:35.000000000 +0800
@@ -101,14 +101,34 @@ static inline void percpu_counter_destro
 
 static inline void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
 {
+#if BITS_PER_LONG == 32
+	preempt_disable();
+	fbc->count = amount;
+	preempt_enable();
+#else
 	fbc->count = amount;
+#endif
+}
+
+static inline s64 percpu_counter_read(struct percpu_counter *fbc)
+{
+#if BITS_PER_LONG == 32
+	s64 count;
+	preempt_disable();
+	count = fbc->count;
+	preempt_enable();
+	return count;
+#else
+	return fbc->count;
+#endif
 }
 
 static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
 {
-	if (fbc->count > rhs)
+	s64 count = percpu_counter_read(fbc);
+	if (count > rhs)
 		return 1;
-	else if (fbc->count < rhs)
+	else if (count < rhs)
 		return -1;
 	else
 		return 0;
@@ -128,18 +148,13 @@ __percpu_counter_add(struct percpu_count
 	percpu_counter_add(fbc, amount);
 }
 
-static inline s64 percpu_counter_read(struct percpu_counter *fbc)
-{
-	return fbc->count;
-}
-
 /*
  * percpu_counter is intended to track positive number. In UP case, the number
  * should never be negative.
  */
 static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
 {
-	return fbc->count;
+	return percpu_counter_read(fbc);
 }
 
 static inline s64 percpu_counter_sum_positive(struct percpu_counter *fbc)



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