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Subject[patch 1/2]block, cfq: fix empty queue crash caused by request merge
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All requests of a queue could be merged to other requests of other queue.
Such queue will not have request in it, but it's in service tree. This
will cause kernel oops.
I encounter a BUG_ON() in cfq_dispatch_request() with next patch, but the
issue should exist without the patch.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
block/cfq-iosched.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

Index: linux/block/cfq-iosched.c
===================================================================
--- linux.orig/block/cfq-iosched.c 2011-12-16 09:01:57.000000000 +0800
+++ linux/block/cfq-iosched.c 2011-12-16 09:07:44.000000000 +0800
@@ -1655,6 +1655,8 @@ cfq_merged_requests(struct request_queue
struct request *next)
{
struct cfq_queue *cfqq = RQ_CFQQ(rq);
+ struct cfq_data *cfqd = q->elevator->elevator_data;
+
/*
* reposition in fifo if next is older than rq
*/
@@ -1669,6 +1671,16 @@ cfq_merged_requests(struct request_queue
cfq_remove_request(next);
cfq_blkiocg_update_io_merged_stats(&(RQ_CFQG(rq))->blkg,
rq_data_dir(next), rq_is_sync(next));
+
+ cfqq = RQ_CFQQ(next);
+ /*
+ * all requests of this queue are merged to other queues, delete it
+ * from the service tree. If it's the active_queue,
+ * cfq_dispatch_requests() will choose to expire it or do idle
+ */
+ if (cfq_cfqq_on_rr(cfqq) && RB_EMPTY_ROOT(&cfqq->sort_list) &&
+ cfqq != cfqd->active_queue)
+ cfq_del_cfqq_rr(cfqd, cfqq);
}

static int cfq_allow_merge(struct request_queue *q, struct request *rq,



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