blk-mq: fix deadlock when reading cpu_list

CPU hotplug handling for blk-mq (blk_mq_queue_reinit) acquires
all_q_mutex in blk_mq_queue_reinit_notify() and then removes sysfs
entries by blk_mq_sysfs_unregister().  Removing sysfs entry needs to
be blocked until the active reference of the kernfs_node to be zero.

On the other hand, reading blk_mq_hw_sysfs_cpu sysfs entry (e.g.
/sys/block/nullb0/mq/0/cpu_list) acquires all_q_mutex in
blk_mq_hw_sysfs_cpus_show().

If these happen at the same time, a deadlock can happen.  Because one
can wait for the active reference to be zero with holding all_q_mutex,
and the other tries to acquire all_q_mutex with holding the active
reference.

The reason that all_q_mutex is acquired in blk_mq_hw_sysfs_cpus_show()
is to avoid reading an imcomplete hctx->cpumask.  Since reading sysfs
entry for blk-mq needs to acquire q->sysfs_lock, we can avoid deadlock
and reading an imcomplete hctx->cpumask by protecting q->sysfs_lock
while hctx->cpumask is being updated.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a5dbd069c9da..31c0c6259c4c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1797,6 +1797,11 @@ static void blk_mq_map_swqueue(struct request_queue *q,
         struct blk_mq_ctx *ctx;
         struct blk_mq_tag_set *set = q->tag_set;
 
+        /*
+         * Avoid others reading imcomplete hctx->cpumask through sysfs
+         */
+        mutex_lock(&q->sysfs_lock);
+
         queue_for_each_hw_ctx(q, hctx, i) {
                 cpumask_clear(hctx->cpumask);
                 hctx->nr_ctx = 0;
@@ -1816,6 +1821,8 @@ static void blk_mq_map_swqueue(struct request_queue *q,
                 hctx->ctxs[hctx->nr_ctx++] = ctx;
         }
 
+        mutex_unlock(&q->sysfs_lock);
+
         queue_for_each_hw_ctx(q, hctx, i) {
                 struct blk_mq_ctxmap *map = &hctx->ctx_map;