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cfq-iosched: Fix a possible race with cfq cgroup removal code

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blkg->key = cfqd is an rcu protected pointer and hence we used to do
call_rcu(cfqd->rcu_head) to free up cfqd after one rcu grace period.

The problem here is that even though cfqd is around, there are no
gurantees that associated request queue (td->queue) or q->queue_lock
is still around. A driver might have called blk_cleanup_queue() and
release the lock.

It might happen that after freeing up the lock we call
blkg->key->queue->queue_ock and crash. This is possible in following
path.

blkiocg_destroy()
 blkio_unlink_group_fn()
  cfq_unlink_blkio_group()

Hence, wait for an rcu peirod if there are groups which have not
been unlinked from blkcg->blkg_list. That way, if there are any groups
which are taking cfq_unlink_blkio_group() path, can safely take queue
lock.

This is how we have taken care of race in throttling logic also.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Vivek Goyal 2011-05-19 15:38:22 -04:00 committed by Jens Axboe
parent 3e59cf9d66
commit 56edf7d75d
1 changed files with 36 additions and 12 deletions
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48
block/cfq-iosched.c
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@ -300,7 +300,9 @@ struct cfq_data {
/* List of cfq groups being managed on this device*/
struct hlist_head cfqg_list;
struct rcu_head rcu;
/* Number of groups which are on blkcg->blkg_list */
unsigned int nr_blkcg_linked_grps;
};
static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
@ -1063,6 +1065,7 @@ static struct cfq_group * cfq_find_alloc_cfqg(struct cfq_data *cfqd,
cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg, (void *)cfqd,
0);
cfqd->nr_blkcg_linked_grps++;
cfqg->weight = blkcg_get_weight(blkcg, cfqg->blkg.dev);
/* Add group on cfqd list */
@ -3815,15 +3818,11 @@ static void cfq_put_async_queues(struct cfq_data *cfqd)
cfq_put_queue(cfqd->async_idle_cfqq);
}
static void cfq_cfqd_free(struct rcu_head *head)
{
kfree(container_of(head, struct cfq_data, rcu));
}
static void cfq_exit_queue(struct elevator_queue *e)
{
struct cfq_data *cfqd = e->elevator_data;
struct request_queue *q = cfqd->queue;
bool wait = false;
cfq_shutdown_timer_wq(cfqd);
@ -3842,7 +3841,13 @@ static void cfq_exit_queue(struct elevator_queue *e)
cfq_put_async_queues(cfqd);
cfq_release_cfq_groups(cfqd);
cfq_blkiocg_del_blkio_group(&cfqd->root_group.blkg);
/*
* If there are groups which we could not unlink from blkcg list,
* wait for a rcu period for them to be freed.
*/
if (cfqd->nr_blkcg_linked_grps)
wait = true;
spin_unlock_irq(q->queue_lock);
@ -3852,8 +3857,20 @@ static void cfq_exit_queue(struct elevator_queue *e)
ida_remove(&cic_index_ida, cfqd->cic_index);
spin_unlock(&cic_index_lock);
/* Wait for cfqg->blkg->key accessors to exit their grace periods. */
call_rcu(&cfqd->rcu, cfq_cfqd_free);
/*
* Wait for cfqg->blkg->key accessors to exit their grace periods.
* Do this wait only if there are other unlinked groups out
* there. This can happen if cgroup deletion path claimed the
* responsibility of cleaning up a group before queue cleanup code
* get to the group.
*
* Do not call synchronize_rcu() unconditionally as there are drivers
* which create/delete request queue hundreds of times during scan/boot
* and synchronize_rcu() can take significant time and slow down boot.
*/
if (wait)
synchronize_rcu();
kfree(cfqd);
}
static int cfq_alloc_cic_index(void)
@ -3909,14 +3926,21 @@ static void *cfq_init_queue(struct request_queue *q)
#ifdef CONFIG_CFQ_GROUP_IOSCHED
/*
* Take a reference to root group which we never drop. This is just
* to make sure that cfq_put_cfqg() does not try to kfree root group
* Set root group reference to 2. One reference will be dropped when
* all groups on cfqd->cfqg_list are being deleted during queue exit.
* Other reference will remain there as we don't want to delete this
* group as it is statically allocated and gets destroyed when
* throtl_data goes away.
*/
cfqg->ref = 1;
cfqg->ref = 2;
rcu_read_lock();
cfq_blkiocg_add_blkio_group(&blkio_root_cgroup, &cfqg->blkg,
(void *)cfqd, 0);
rcu_read_unlock();
cfqd->nr_blkcg_linked_grps++;
/* Add group on cfqd->cfqg_list */
hlist_add_head(&cfqg->cfqd_node, &cfqd->cfqg_list);
#endif
/*
* Not strictly needed (since RB_ROOT just clears the node and we