CFQ is broken for CONFIG_BLK_CGROUP=y, CFQ_GROUP_IOSCHED=n

From: Dmitry Monakhov
Date: Wed Apr 28 2010 - 08:45:33 EST



I've had an oops on kernel boot due to NULL pointer deference
linux-2.6-block/for-next HEAD:7eaed1226ab411ee5dc8c34fc0d8034e4c98e3c6
I've enabled CONFIG_BLK_CGROUP, but not CFQ_GROUP_IOSCHED
In this case cfq_ref_get_cfqg() defined as
static inline struct cfq_group *cfq_ref_get_cfqg(struct cfq_group *cfqg)
{
return NULL;
}
So following call trace is simply NOOP
cfq_set_request()
rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);

Which later result in OOPS on bio insertion
cfq_insert_request
-> blkiocg_update_io_add_stats(&(RQ_CFQG(rq))->blkg,...)
-> spin_lock_irqsave(&blkg->stats_lock, flags);

Bad commit.