[ 3.5.y.z extended stable ] Patch "vgacon.c: add cond reschedule points in vgacon_do_font_op" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    vgacon.c: add cond reschedule points in vgacon_do_font_op

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 757d9c6d7ae61c4e3a2762f8196cf98e3a2d2d97 Mon Sep 17 00:00:00 2001
From: Marcelo Tosatti <mtosatti at redhat.com>
Date: Thu, 20 Jun 2013 18:05:56 -0300
Subject: [PATCH] vgacon.c: add cond reschedule points in vgacon_do_font_op

commit 7e6d72c15ff4cc0c27573901bb05f9eddbd71ed4 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 at redhat.com>
Signed-off-by: Dave Airlie <airlied at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/video/console/vgacon.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 5855d17..0dd6d96 100644
--- 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();
+				}
 		}
 	}

--
1.8.1.2