blk-iolatency: Fix inflight count imbalances and IO hangs on offline

In the Linux kernel, the following vulnerability has been resolved:

blk-iolatency: Fix inflight count imbalances and IO hangs on offline

iolatency needs to track the number of inflight IOs per cgroup. As this

tracking can be expensive, it is disabled when no cgroup has iolatency

configured for the device. To ensure that the inflight counters stay

balanced, iolatency_set_limit() freezes the request_queue while manipulating

the enabled counter, which ensures that no IO is in flight and thus all

counters are zero.

Unfortunately, iolatency_set_limit() isn't the only place where the enabled

counter is manipulated. iolatency_pd_offline() can also dec the counter and

trigger disabling. As this disabling happens without freezing the q, this

can easily happen while some IOs are in flight and thus leak the counts.

This can be easily demonstrated by turning on iolatency on an one empty

cgroup while IOs are in flight in other cgroups and then removing the

cgroup. Note that iolatency shouldn't have been enabled elsewhere in the

system to ensure that removing the cgroup disables iolatency for the whole

device.

The following keeps flipping on and off iolatency on sda:

echo +io > /sys/fs/cgroup/cgroup.subtree_control

while true; do

mkdir -p /sys/fs/cgroup/test

echo '8:0 target=100000' > /sys/fs/cgroup/test/io.latency

sleep 1

rmdir /sys/fs/cgroup/test

sleep 1

done

and there's concurrent fio generating direct rand reads:

fio --name test --filename=/dev/sda --direct=1 --rw=randread \

--runtime=600 --time_based --iodepth=256 --numjobs=4 --bs=4k

while monitoring with the following drgn script:

while True:

for css in css_for_each_descendant_pre(prog['blkcg_root'].css.address_of_()):

for pos in hlist_for_each(container_of(css, 'struct blkcg', 'css').blkg_list):

blkg = container_of(pos, 'struct blkcg_gq', 'blkcg_node')

pd = blkg.pd[prog['blkcg_policy_iolatency'].plid]

if pd.value_() == 0:

continue

iolat = container_of(pd, 'struct iolatency_grp', 'pd')

inflight = iolat.rq_wait.inflight.counter.value_()

if inflight:

print(f'inflight={inflight} {disk_name(blkg.q.disk).decode("utf-8")} '

f'{cgroup_path(css.cgroup).decode("utf-8")}')

time.sleep(1)

The monitoring output looks like the following:

inflight=1 sda /user.slice

inflight=1 sda /user.slice

...

inflight=14 sda /user.slice

inflight=13 sda /user.slice

inflight=17 sda /user.slice

inflight=15 sda /user.slice

inflight=18 sda /user.slice

inflight=17 sda /user.slice

inflight=20 sda /user.slice

inflight=19 sda /user.slice <- fio stopped, inflight stuck at 19

inflight=19 sda /user.slice

inflight=19 sda /user.slice

If a cgroup with stuck inflight ends up getting throttled, the throttled IOs

will never get issued as there's no completion event to wake it up leading

to an indefinite hang.

This patch fixes the bug by unifying enable handling into a work item which

is automatically kicked off from iolatency_set_min_lat_nsec() which is

called from both iolatency_set_limit() and iolatency_pd_offline() paths.

Punting to a work item is necessary as iolatency_pd_offline() is called

under spinlocks while freezing a request_queue requires a sleepable context.

This also simplifies the code reducing LOC sans the comments and avoids the

unnecessary freezes which were happening whenever a cgroup's latency target

is newly set or cleared.