Messages in this thread Patch in this message | | | From | Akinobu Mita <> | Subject | [PATCH v2 6/6] blk-mq: fix deadlock when reading cpu_list | Date | Thu, 2 Jul 2015 23:29:57 +0900 |
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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> Cc: Jens Axboe <axboe@kernel.dk> Cc: Ming Lei <tom.leiming@gmail.com> --- block/blk-mq-sysfs.c | 4 ---- block/blk-mq.c | 17 +++++++---------- block/blk-mq.h | 2 -- 3 files changed, 7 insertions(+), 16 deletions(-)
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c index 8448513..9549020 100644 --- a/block/blk-mq-sysfs.c +++ b/block/blk-mq-sysfs.c @@ -218,8 +218,6 @@ 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); @@ -229,8 +227,6 @@ 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; } diff --git a/block/blk-mq.c b/block/blk-mq.c index f31de35..76fd825 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1815,6 +1815,11 @@ 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; @@ -1835,6 +1840,8 @@ 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; @@ -2321,16 +2328,6 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr) return ret; } -void blk_mq_disable_hotplug(void) -{ - mutex_lock(&all_q_mutex); -} - -void blk_mq_enable_hotplug(void) -{ - mutex_unlock(&all_q_mutex); -} - static int __init blk_mq_init(void) { blk_mq_cpu_init(); diff --git a/block/blk-mq.h b/block/blk-mq.h index f4fea79..2592682 100644 --- a/block/blk-mq.h +++ b/block/blk-mq.h @@ -44,8 +44,6 @@ void blk_mq_init_cpu_notifier(struct blk_mq_cpu_notifier *notifier, void blk_mq_register_cpu_notifier(struct blk_mq_cpu_notifier *notifier); void blk_mq_unregister_cpu_notifier(struct blk_mq_cpu_notifier *notifier); void blk_mq_cpu_init(void); -void blk_mq_enable_hotplug(void); -void blk_mq_disable_hotplug(void); /* * CPU -> queue mappings -- 1.9.1
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