powerpc: Update DSCR on all CPUs when writing sysfs dscr_default

commit 1b6ca2a6 upstream.

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:

http://ozlabs.org/~anton/junkcode/dscr_explicit_test.cSigned-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/powerpc/kernel/sysfs.c

@@ -192,6 +192,12 @@ static ssize_t show_dscr_default(struct sysdev_class *class,
 	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 sysdev_class *class,
 		struct sysdev_class_attribute *attr, const char *buf,
 		size_t count)
@@ -204,6 +210,8 @@ static ssize_t __used store_dscr_default(struct sysdev_class *class,
 		return -EINVAL;
 	dscr_default = val;
 
+	on_each_cpu(update_dscr, NULL, 1);
+
 	return count;
 }