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authorAkinobu Mita <akinobu.mita@gmail.com>2015-09-26 20:09:25 +0300
committerJens Axboe <axboe@fb.com>2015-09-29 20:32:51 +0300
commit60de074ba1e8f327db19bc33d8530131ac01695d (patch)
treeb23080dbac15be404a4c47da9d83ec7b83cbd3f7 /block/blk-mq.c
parent5778322e67ed34dc9f391a4a5cbcbb856071ceba (diff)
downloadlinux-60de074ba1e8f327db19bc33d8530131ac01695d.tar.xz
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>
Diffstat (limited to 'block/blk-mq.c')
-rw-r--r--block/blk-mq.c7
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;