virtio_balloon: don't softlockup on huge balloon changes.

When adding or removing 100G from a balloon:

    BUG: soft lockup - CPU#0 stuck for 22s! [vballoon:367]

We have a wait_event_interruptible(), but the condition is always true
(more ballooning to do) so we don't ever sleep.  We also have a
wait_event() for the host to ack, but that is also always true as QEMU
is synchronous for balloon operations.

Reported-by: Gopesh Kumar Chaudhary <gopchaud@in.ibm.com>
Cc: stable@kernel.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 34bdabaecbd6..36e7859a31aa 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -310,6 +310,12 @@ static int balloon(void *_vballoon)
                 else if (diff < 0)
                         leak_balloon(vb, -diff);
                 update_balloon_size(vb);
+
+                /*
+                 * For large balloon changes, we could spend a lot of time
+                 * and always have work to do.  Be nice if preempt disabled.
+                 */
+                cond_resched();
         }
         return 0;
 }