vgacon.c: add cond reschedule points in vgacon_do_font_op

commit 7e6d72c1 upstream.

Booting a 64-vcpu KVM guest, with CONFIG_PREEMPT_VOLUNTARY,
can result in a soft lockup:

BUG: soft lockup - CPU#41 stuck for 67s! [setfont:1505]
 RIP: 0010:[<ffffffff812c48da>]
[<ffffffff812c48da>] vgacon_do_font_op.clone.0+0x1ba/0x550

This is due to the 8192 (cmapsz) IO operations taking longer than expected
due to lock contention in QEMU.

Add conditional resched points in between writes allowing other tasks to
execute.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[bwh: Backported to 3.2: add #include <linux/sched.h>, already present
 upstream]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/video/console/vgacon.c

@@ -42,6 +42,7 @@
 #include <linux/kd.h>
 #include <linux/slab.h>
 #include <linux/vt_kern.h>
+#include <linux/sched.h>
 #include <linux/selection.h>
 #include <linux/spinlock.h>
 #include <linux/ioport.h>
@@ -1124,11 +1125,15 @@ static int vgacon_do_font_op(struct vgastate *state,char *arg,int set,int ch512)
 
 	if (arg) {
 		if (set)
-			for (i = 0; i < cmapsz; i++)
+			for (i = 0; i < cmapsz; i++) {
 				vga_writeb(arg[i], charmap + i);
+				cond_resched();
+			}
 		else
-			for (i = 0; i < cmapsz; i++)
+			for (i = 0; i < cmapsz; i++) {
 				arg[i] = vga_readb(charmap + i);
+				cond_resched();
+			}
 
 		/*
 		 * In 512-character mode, the character map is not contiguous if
@@ -1139,11 +1144,15 @@ static int vgacon_do_font_op(struct vgastate *state,char *arg,int set,int ch512)
 			charmap += 2 * cmapsz;
 			arg += cmapsz;
 			if (set)
-				for (i = 0; i < cmapsz; i++)
+				for (i = 0; i < cmapsz; i++) {
 					vga_writeb(arg[i], charmap + i);
+					cond_resched();
+				}
 			else
-				for (i = 0; i < cmapsz; i++)
+				for (i = 0; i < cmapsz; i++) {
 					arg[i] = vga_readb(charmap + i);
+					cond_resched();
+				}
 		}
 	}