[PATCH V2 0/8]: blk-mq: use static_rqs to iterate busy tags

Jianchao Wang
2019-03-25T05:47:29Z

As we know, there is a risk of accesing stale requests when iterate
in-flight requests with tags->rqs[] and this has been talked in following
thread,
[1] https://marc.info/?l=linux-scsi&m=154511693912752&w=2
[2] https://marc.info/?l=linux-block&m=154526189023236&w=2

A typical sence could be
blk_mq_get_request blk_mq_queue_tag_busy_iter
-> blk_mq_get_tag
-> bt_for_each
-> bt_iter
-> rq = taags->rqs[]
-> rq->q
-> blk_mq_rq_ctx_init
-> data->hctx->tags->rqs[rq->tag] = rq;

The root cause is that there is a window between set bit on tag sbitmap
and set tags->rqs[].

This patch would fix this issue by iterating requests with tags->static_rqs[]
instead of tags->rqs[] which would be changed dynamically. Moreover,
we will try to get a non-zero q_usage_counter before access hctxs and tags and
thus could avoid the race with updating nr_hw_queues, switching io scheduler
and even queue clean up which are all under a frozen and drained queue.

The 1st patch get rid of the useless of synchronize_rcu in __blk_mq_update_nr_hw_queues

The 2nd patch modify the blk_mq_queue_tag_busy_iter to use tags->static_rqs[]
instead of tags->rqs[] to iterate the busy tags.

The 3rd ~ 7th patch change the blk_mq_tagset_busy_iter to blk_mq_queue_tag_busy_iter
which is safer

The 8th patch get rid of the blk_mq_tagset_busy_iter.

Change log

V1 -> V2:
- Add wrapper to hide "inflight" parameter to user based on Sagi's suggestion.
- Other misc changes on comment.

Jianchao Wang (8)
blk-mq: get rid of the synchronize_rcu in
blk-mq: use static_rqs instead of rqs to iterate tags
blk-mq: use blk_mq_queue_tag_inflight_iter in debugfs
mtip32xx: use blk_mq_queue_tag_inflight_iter
nbd: use blk_mq_queue_tag_inflight_iter
skd: use blk_mq_queue_tag_inflight_iter
nvme: use blk_mq_queue_tag_inflight_iter
blk-mq: remove blk_mq_tagset_busy_iter

diff stat

block/blk-mq-debugfs.c | 2 +-
block/blk-mq-tag.c | 193 ++++++++++++++------------------------
block/blk-mq-tag.h | 4 +-
block/blk-mq.c | 31 ++----
drivers/block/mtip32xx/mtip32xx.c | 6 +-
drivers/block/nbd.c | 2 +-
drivers/block/skd_main.c | 4 +-
drivers/nvme/host/core.c | 12 +++
drivers/nvme/host/fc.c | 10 +-
drivers/nvme/host/nvme.h | 2 +
drivers/nvme/host/pci.c | 5 +-
drivers/nvme/host/rdma.c | 4 +-
drivers/nvme/host/tcp.c | 5 +-
drivers/nvme/target/loop.c | 4 +-
include/linux/blk-mq.h | 7 +-
15 files changed, 119 insertions(+), 172 deletions(-)

Thanks
Jianchao