blkcg: skip blkg printing if q isn't associated with disk

blk-cgroup printing code currently assumes that there is a device/disk
associated with every queue in the system, but modules like floppy,
can instantiate request queues without registering disk which can lead
to oops.

Skip the queue/blkg which don't have dev/disk associated with them.

-tj: Factored out backing_dev_info check into blkg_dev_name().

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 14 insertions, 3 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 8742af3be84b..76942360872b 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -951,13 +951,24 @@ static int blkiocg_file_write(struct cgroup *cgrp, struct cftype *cft,
         return ret;
 }
 
+static const char *blkg_dev_name(struct blkio_group *blkg)
+{
+        /* some drivers (floppy) instantiate a queue w/o disk registered */
+        if (blkg->q->backing_dev_info.dev)
+                return dev_name(blkg->q->backing_dev_info.dev);
+        return NULL;
+}
+
 static void blkio_print_group_conf(struct cftype *cft, struct blkio_group *blkg,
                                    struct seq_file *m)
 {
-        const char *dname = dev_name(blkg->q->backing_dev_info.dev);
+        const char *dname = blkg_dev_name(blkg);
         int fileid = BLKIOFILE_ATTR(cft->private);
         int rw = WRITE;
 
+        if (!dname)
+                return;
+
         switch (blkg->plid) {
                 case BLKIO_POLICY_PROP:
                         if (blkg->conf.weight)
@@ -1049,9 +1060,9 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
 
         rcu_read_lock();
         hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node) {
-                const char *dname = dev_name(blkg->q->backing_dev_info.dev);
+                const char *dname = blkg_dev_name(blkg);
 
-                if (BLKIOFILE_POLICY(cft->private) != blkg->plid)
+                if (!dname || BLKIOFILE_POLICY(cft->private) != blkg->plid)
                         continue;
                 if (pcpu)
                         cgroup_total += blkio_get_stat_cpu(blkg, cb, dname,