powerpc: Update DSCR on all CPUs when writing sysfs dscr_default

Writing to dscr_default in sysfs doesn't actually change the DSCR -
we rely on a context switch on each CPU to do the work. There is no
guarantee we will get a context switch in a reasonable amount of time
so fire off an IPI to force an immediate change.

This issue was found with the following test case:


Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org> # 3.0+
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
index 3529446..d4cbbd1 100644
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -194,6 +194,12 @@
 	return sprintf(buf, "%lx\n", dscr_default);
+static void update_dscr(void *dummy)
+	if (!current->thread.dscr_inherit)
+		mtspr(SPRN_DSCR, dscr_default);
 static ssize_t __used store_dscr_default(struct device *dev,
 		struct device_attribute *attr, const char *buf,
 		size_t count)
@@ -206,6 +212,8 @@
 		return -EINVAL;
 	dscr_default = val;
+	on_each_cpu(update_dscr, NULL, 1);
 	return count;