Re: [PATCH]atomic_inc_return() for i386/x86_64 (Re: RCU issue with SELinux)

["Kaigai Kohei" <kaigai@ak.jp.nec.com>]

[-- Attachment #1: Type: text/plain, Size: 945 bytes --]

Hi Andi, thanks for your comment.

> > We can avoid the problem by the simple solution, since SMP
> > on 386 boxes isn't supported. It is to disable interrupt
> > while updating atomic_t variable.
> 
> The patch is wrong.  A CONFIG_M386 kernel can run on non
> 386 SMP boxes. Your patch would be racy then. The only thing 
> that's not supported is a real 386 with multiple CPUs.
> 
> You either have to check boot_cpu_data.x86 == 3 at runtime or 
> use alternative() like I earlier suggested.

Indeed, the attached patch is implemented according to the suggestion.
The code for legacy-386 is used only when boot_cpu_data.x86 == 3.
This run-time check is done only when CONFIG_M386 is set.
Otherwise, we use 'XADD' operation for atomic_add_return()
as the previous patch.

I don't adopt the alternative(), because the macro doesn't fit for the situation
when arguments for __asm__ are necessary.

Thanks.
--------
Kai Gai <kaigai@ak.jp.nec.com>

[-- Attachment #2: atomic_inc_return-2.6.8.1.M386.patch --]
[-- Type: application/octet-stream, Size: 3270 bytes --]

--- linux-2.6.8.1/include/asm-i386/atomic.h	2004-08-14 19:55:09.000000000 +0900
+++ linux-2.6.8.1.rcu/include/asm-i386/atomic.h	2004-09-01 14:40:40.000000000 +0900
@@ -1,8 +1,10 @@
 #ifndef __ARCH_I386_ATOMIC__
 #define __ARCH_I386_ATOMIC__
 
 #include <linux/config.h>
+#include <linux/compiler.h>
+#include <asm/processor.h>
 
 /*
  * Atomic operations that C can't guarantee us.  Useful for
  * resource counting etc..
@@ -175,8 +177,48 @@
 		:"ir" (i), "m" (v->counter) : "memory");
 	return c;
 }
 
+/**
+ * atomic_add_return - add and return
+ * @v: pointer of type atomic_t
+ * @i: integer value to add
+ *
+ * Atomically adds @i to @v and returns @i + @v
+ */
+static __inline__ int atomic_add_return(int i, atomic_t *v)
+{
+	int __i;
+#ifdef CONFIG_M386
+	if(unlikely(boot_cpu_data.x86==3))
+		goto no_xadd;
+#endif
+	/* Modern 486+ processor */
+	__i = i;
+	__asm__ __volatile__(
+		LOCK "xaddl %0, %1;"
+		:"=r"(i)
+		:"m"(v->counter), "0"(i));
+	return i + __i;
+
+#ifdef CONFIG_M386
+no_xadd: /* Legacy 386 processor */
+	local_irq_disable();
+	__i = atomic_read(v);
+	atomic_set(v, i + __i);
+	local_irq_enable();
+	return i + __i;
+#endif
+}
+
+static __inline__ int atomic_sub_return(int i, atomic_t *v)
+{
+	return atomic_add_return(-i,v);
+}
+
+#define atomic_inc_return(v)  (atomic_add_return(1,v))
+#define atomic_dec_return(v)  (atomic_sub_return(1,v))
+
 /* These are x86-specific, used by some header files */
 #define atomic_clear_mask(mask, addr) \
 __asm__ __volatile__(LOCK "andl %0,%1" \
 : : "r" (~(mask)),"m" (*addr) : "memory")
--- linux-2.6.8.1/include/asm-x86_64/atomic.h	2004-08-14 19:56:23.000000000 +0900
+++ linux-2.6.8.1.rcu/include/asm-x86_64/atomic.h	2004-08-25 11:57:36.000000000 +0900
@@ -177,8 +177,33 @@
 		:"ir" (i), "m" (v->counter) : "memory");
 	return c;
 }
 
+/**
+ * atomic_add_return - add and return
+ * @v: pointer of type atomic_t
+ * @i: integer value to add
+ *
+ * Atomically adds @i to @v and returns @i + @v
+ */
+static __inline__ int atomic_add_return(int i, atomic_t *v)
+{
+	int __i = i;
+	__asm__ __volatile__(
+		LOCK "xaddl %0, %1;"
+		:"=r"(i)
+		:"m"(v->counter), "0"(i));
+	return i + __i;
+}
+
+static __inline__ int atomic_sub_return(int i, atomic_t *v)
+{
+	return atomic_add_return(-i,v);
+}
+
+#define atomic_inc_return(v)  (atomic_add_return(1,v))
+#define atomic_dec_return(v)  (atomic_sub_return(1,v))
+
 /* These are x86-specific, used by some header files */
 #define atomic_clear_mask(mask, addr) \
 __asm__ __volatile__(LOCK "andl %0,%1" \
 : : "r" (~(mask)),"m" (*addr) : "memory")
--- linux-2.6.8.1/include/asm-arm/atomic.h	2004-08-14 19:54:50.000000000 +0900
+++ linux-2.6.8.1.rcu/include/asm-arm/atomic.h	2004-08-24 19:31:56.000000000 +0900
@@ -194,8 +194,10 @@
 #define atomic_dec(v)		atomic_sub(1, v)
 
 #define atomic_inc_and_test(v)	(atomic_add_return(1, v) == 0)
 #define atomic_dec_and_test(v)	(atomic_sub_return(1, v) == 0)
+#define atomic_inc_return(v)    (atomic_add_return(1, v))
+#define atomic_dec_return(v)    (atomic_sub_return(1, v))
 
 #define atomic_add_negative(i,v) (atomic_add_return(i, v) < 0)
 
 /* Atomic operations are already serializing on ARM */