s390/vdso: fix getcpu

getcpu reads the required values for cpu and node with two
instructions. This might lead to an inconsistent result if user space
gets preempted and migrated to a different CPU between the two
instructions.

Fix this by using just a single instruction to read both values at
once.

This is currently rather a theoretical bug, since there is no real
NUMA support available (except for NUMA emulation).

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/vdso.h

@@ -41,8 +41,17 @@ struct vdso_data {
 struct vdso_per_cpu_data {
 	__u64 ectg_timer_base;
 	__u64 ectg_user_time;
-	__u32 cpu_nr;
-	__u32 node_id;
+	/*
+	 * Note: node_id and cpu_nr must be at adjacent memory locations.
+	 * VDSO userspace must read both values with a single instruction.
+	 */
+	union {
+		__u64 getcpu_val;
+		struct {
+			__u32 node_id;
+			__u32 cpu_nr;
+		};
+	};
 };
 
 extern struct vdso_data *vdso_data;

arch/s390/kernel/asm-offsets.c

@@ -78,8 +78,7 @@ int main(void)
 	OFFSET(__VDSO_TS_END, vdso_data, ts_end);
 	OFFSET(__VDSO_ECTG_BASE, vdso_per_cpu_data, ectg_timer_base);
 	OFFSET(__VDSO_ECTG_USER, vdso_per_cpu_data, ectg_user_time);
-	OFFSET(__VDSO_CPU_NR, vdso_per_cpu_data, cpu_nr);
-	OFFSET(__VDSO_NODE_ID, vdso_per_cpu_data, node_id);
+	OFFSET(__VDSO_GETCPU_VAL, vdso_per_cpu_data, getcpu_val);
 	BLANK();
 	/* constants used by the vdso */
 	DEFINE(__CLOCK_REALTIME, CLOCK_REALTIME);

arch/s390/kernel/vdso32/getcpu.S

@@ -16,10 +16,8 @@
 	.type  __kernel_getcpu,@function
 __kernel_getcpu:
 	CFI_STARTPROC
-	la	%r4,0
 	sacf	256
-	l	%r5,__VDSO_CPU_NR(%r4)
-	l	%r4,__VDSO_NODE_ID(%r4)
+	lm	%r4,%r5,__VDSO_GETCPU_VAL(%r0)
 	sacf	0
 	ltr	%r2,%r2
 	jz	2f

arch/s390/kernel/vdso64/getcpu.S

@@ -16,10 +16,8 @@
 	.type  __kernel_getcpu,@function
 __kernel_getcpu:
 	CFI_STARTPROC
-	la	%r4,0
 	sacf	256
-	l	%r5,__VDSO_CPU_NR(%r4)
-	l	%r4,__VDSO_NODE_ID(%r4)
+	lm	%r4,%r5,__VDSO_GETCPU_VAL(%r0)
 	sacf	0
 	ltgr	%r2,%r2
 	jz	2f