vgacon.c: add cond reschedule points in vgacon_do_font_op

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>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 5855d17d19ac9b8d493c13d7f0dd879ccf0710e4..0dd6d9625bb51ab11f1ff3da3123e57d9d60f3ce 100644 (file)
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -1124,11 +1124,15 @@ 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 +1143,15 @@ 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();
+                               }
                }
        }