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authorAkinobu Mita <akinobu.mita@gmail.com>2015-09-27 02:09:25 +0900
committerJens Axboe <axboe@fb.com>2015-09-29 11:32:51 -0600
commit60de074ba1e8f327db19bc33d8530131ac01695d (patch)
treeb23080dbac15be404a4c47da9d83ec7b83cbd3f7 /block/blk-mq-sysfs.c
parent5778322e67ed34dc9f391a4a5cbcbb856071ceba (diff)
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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-sysfs.c')
-rw-r--r--block/blk-mq-sysfs.c4
1 files changed, 0 insertions, 4 deletions
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 189f5ae6522a..788fffd9b409 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -229,8 +229,6 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
unsigned int i, first = 1;
ssize_t ret = 0;
- blk_mq_disable_hotplug();
-
for_each_cpu(i, hctx->cpumask) {
if (first)
ret += sprintf(ret + page, "%u", i);
@@ -240,8 +238,6 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
first = 0;
}
- blk_mq_enable_hotplug();
-
ret += sprintf(ret + page, "\n");
return ret;
}