[PATCH 2/4] cfq-iosched: fix no-idle preemption logic

[Corrado Zoccolo <czoccolo@gmail.com>]

An incoming no-idle queue should preempt the active no-idle queue
only if the active queue is idling due to service tree empty.
Previous code was buggy in two ways:
* it relied on service_tree field to be set on the active queue, while
  it is not set when the code is idling for a new request
* it didn't check for the service tree empty condition, so could lead to
  LIFO behaviour if multiple queues with depth > 1 were preempting each
  other on an non-NCQ device.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
---
 block/cfq-iosched.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6925ab9..2a304f4 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2401,8 +2401,9 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
 	if (cfq_class_idle(cfqq))
 		return true;
 
-	if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD
-	    && new_cfqq->service_tree == cfqq->service_tree)
+	if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
+	    cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
+	    new_cfqq->service_tree->count == 1)
 		return true;
 
 	/*
-- 
1.6.2.5