dm cache policy smq: fix missing locks in invalidating cache blocks

dm cache policy smq: fix missing locks in invalidating cache blocks

In passthrough mode, the policy invalidate_mapping operation is called
simultaneously from multiple workers, thus it should be protected by a
lock. Otherwise, we might end up with data races on the allocated blocks
counter, or even use-after-free issues with internal data structures
when doing concurrent writes.

Note that the existing FIXME in smq_invalidate_mapping() doesn't affect
passthrough mode since migration tasks don't exist there, but would need
attention if supporting fast device shrinking via suspend/resume without
target reloading.

Reproduce steps:

1. Create a cache device consisting of 1024 cache entries

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 131072 linear /dev/sdc 8192"
dmsetup create corig --table "0 262144 linear /dev/sdc 262144"
dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct
dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0"

2. Populate the cache, and record the number of cached blocks

fio --name=populate --filename=/dev/mapper/cache --rw=randwrite --bs=4k \
--size=64m --direct=1
nr_cached=$(dmsetup status cache | awk '{split($7, a, "/"); print a[1]}')

3. Reload the cache into passthrough mode

dmsetup suspend cache
dmsetup reload cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0"
dmsetup resume cache

4. Write to the passthrough cache. By setting multiple jobs with I/O
   size equal to the cache block size, cache blocks are invalidated
   concurrently from different workers.

fio --filename=/dev/mapper/cache --name=test --rw=randwrite --bs=64k \
--direct=1 --numjobs=2 --randrepeat=0 --size=64m

5. Check if demoted matches cached block count. These numbers should
   match but may differ due to the data race.

nr_demoted=$(dmsetup status cache | awk '{print $12}')
echo "$nr_cached, $nr_demoted"

Fixes: b29d4986d0da ("dm cache: significant rework to leverage dm-bio-prison-v2")
Signed-off-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>