Subject: Re: Kernel crash after using new Intel NIC (igb)

From: Arun Sharma <[email-masked]>
Date: Fri, 27 May 2011 14:14:19 -0700

On Fri, May 27, 2011 at 09:56:59PM +0200, Eric Dumazet wrote:
> > 
> > This looks very similar to atomic_add_unless(). If we had a
> > 
> > __atomic_add_unless() that returned "old", we could then do:
> > 
> > atomic_add_unless() { return __atomic_add_unless() != u }
> > atomic_add_unless_return() { return __atomic_add_unless() + a}
> > 
> 
> Sure !
> 
> I preferred to not touch lot of files in kernel (atomic_add_unless() is
> defined in several files) because its a stable candidate patch (2.6.36+)
> 
> So a cleanup patch for 2.6.40+ is certainly doable, do you want to do
> this ?

The attached works for me for x86_64. Cc'ing Ingo/Thomas for comment.

 -Arun

atomic: Refactor atomic_add_unless

Commit 686a7e3 (inetpeer: fix race in unused_list manipulations)
in net-2.6 added a atomic_add_unless_return() variant that tries
to detect 0->1 transitions of an atomic reference count.

This sounds like a generic functionality that could be expressed
in terms of an __atomic_add_unless() that returned the old value
instead of a bool.

Signed-off-by: Arun Sharma <[email-masked]>
---
 arch/x86/include/asm/atomic.h |   22 ++++++++++++++++++----
 1 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
index 952a826..bbdbffe 100644
--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -221,15 +221,15 @@ static inline int atomic_xchg(atomic_t *v, int new)
 }
 
 /**
- * atomic_add_unless - add unless the number is already a given value
+ * __atomic_add_unless - add unless the number is already a given value
  * @v: pointer of type atomic_t
  * @a: the amount to add to v...
  * @u: ...unless v is equal to u.
  *
  * Atomically adds @a to @v, so long as @v was not already @u.
- * Returns non-zero if @v was not @u, and zero otherwise.
+ * Returns the old value of v
  */
-static inline int atomic_add_unless(atomic_t *v, int a, int u)
+static inline int __atomic_add_unless(atomic_t *v, int a, int u)
 {
 	int c, old;
 	c = atomic_read(v);
@@ -241,7 +241,21 @@ static inline int atomic_add_unless(atomic_t *v, int a, int u)
 			break;
 		c = old;
 	}
-	return c != (u);
+	return c;
+}
+
+/**
+ * atomic_add_unless - add unless the number is already a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as @v was not already @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	return __atomic_add_unless(v, a, u) != u;
 }
 
 #define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
-- 
1.7.4