[PATCH 6/7] blk-mq: quiesce queue via percpu_ref

Ming Lei
2017-05-25T04:22:08Z

One big problem of blk_mq_quiesce_queue() is that it
can't prevent .queue_rq() in direct issue path from
being run even though hw queues are stopped.

It is observed that request double-free/use-after-free
can be triggered easily when canceling NVMe requests via
blk_mq_tagset_busy_iter(...nvme_cancel_request) in nvme_dev_disable().
The reason is that blk_mq_quiesce_queue() doesn't prevent
dispatch from being run.

Actually other drivers(mtip32xx, nbd) need to quiesce
queue first before canceling dispatched requests via
blk_mq_tagset_busy_iter(), otherwise use-after-free
can be caused.

This patch implements queue quiescing via percpu_ref,
and fixes the above issue, also has the following benefits:

- rcu & srcu are used for non-blocking and blocking respectively,
code becomes much clean after we unify both via percpu_ref

- the fat 'srcu_struct' can be removed from 'blk_mq_hw_ctx'

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq.c | 112 +++++++++++++++++++++++++++++++------------------
include/linux/blk-mq.h | 2 -
include/linux/blkdev.h | 5 +++
3 files changed, 77 insertions(+), 42 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a26fee3fb389..6316efb42e04 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -164,20 +164,23 @@ EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
*/
void blk_mq_quiesce_queue(struct request_queue *q)
{
- struct blk_mq_hw_ctx *hctx;
- unsigned int i;
- bool rcu = false;
+ bool old;

__blk_mq_stop_hw_queues(q, true);

- queue_for_each_hw_ctx(q, hctx, i) {
- if (hctx->flags & BLK_MQ_F_BLOCKING)
- synchronize_srcu(&hctx->queue_rq_srcu);
- else
- rcu = true;
- }
- if (rcu)
- synchronize_rcu();
+ mutex_lock(&q->quiesce_mutex);
+
+ spin_lock_irq(q->queue_lock);
+ old = queue_flag_test_and_set(QUEUE_FLAG_QUIESCED, q);
+ spin_unlock_irq(q->queue_lock);
+
+ if (old)
+ goto exit;
+
+ percpu_ref_kill(&q->q_dispatch_counter);
+ wait_event(q->quiesce_wq, percpu_ref_is_zero(&q->q_dispatch_counter));
+ exit:
+ mutex_unlock(&q->quiesce_mutex);
}
EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);

@@ -190,6 +193,21 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
*/
void blk_mq_unquiesce_queue(struct request_queue *q)
{
+ bool old;
+
+ mutex_lock(&q->quiesce_mutex);
+
+ spin_lock_irq(q->queue_lock);
+ old = queue_flag_test_and_clear(QUEUE_FLAG_QUIESCED, q);
+ spin_unlock_irq(q->queue_lock);
+
+ if (!old)
+ goto exit;
+
+ percpu_ref_reinit(&q->q_dispatch_counter);
+ exit:
+ mutex_unlock(&q->quiesce_mutex);
+
blk_mq_start_stopped_hw_queues(q, true);
}
EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
@@ -209,6 +227,9 @@ void blk_mq_wake_waiters(struct request_queue *q)
* the queue are notified as well.
*/
wake_up_all(&q->mq_freeze_wq);
+
+ /* Forcibly unquiesce the queue to avoid having stuck requests */
+ blk_mq_unquiesce_queue(q);
}

bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)
@@ -1099,24 +1120,28 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
return (queued + errors) != 0;
}

-static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
+static inline bool blk_mq_dispatch_start(struct request_queue *q)
+{
+ return percpu_ref_tryget_live(&q->q_dispatch_counter);
+}
+
+static inline void blk_mq_dispatch_end(struct request_queue *q)
{
- int srcu_idx;
+ percpu_ref_put(&q->q_dispatch_counter);
+}

+static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
+{
WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
cpu_online(hctx->next_cpu));

- if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
- rcu_read_lock();
- blk_mq_sched_dispatch_requests(hctx);
- rcu_read_unlock();
- } else {
- might_sleep();
+ if (!blk_mq_dispatch_start(hctx->queue))
+ return;

- srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
- blk_mq_sched_dispatch_requests(hctx);
- srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
- }
+ might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING);
+ blk_mq_sched_dispatch_requests(hctx);
+
+ blk_mq_dispatch_end(hctx->queue);
}

/*
@@ -1519,18 +1544,14 @@ static void __blk_mq_try_issue_directly(struct request *rq, blk_qc_t *cookie,
static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
struct request *rq, blk_qc_t *cookie)
{
- if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
- rcu_read_lock();
- __blk_mq_try_issue_directly(rq, cookie, false);
- rcu_read_unlock();
- } else {
- unsigned int srcu_idx;
-
- might_sleep();
+ bool blocking = hctx->flags & BLK_MQ_F_BLOCKING;

- srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
- __blk_mq_try_issue_directly(rq, cookie, true);
- srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
+ if (blk_mq_dispatch_start(hctx->queue)) {
+ might_sleep_if(blocking);
+ __blk_mq_try_issue_directly(rq, cookie, blocking);
+ blk_mq_dispatch_end(hctx->queue);
+ } else {
+ blk_mq_sched_insert_request(rq, false, false, false, blocking);
}
}

@@ -1869,9 +1890,6 @@ static void blk_mq_exit_hctx(struct request_queue *q,
if (set->ops->exit_hctx)
set->ops->exit_hctx(hctx, hctx_idx);

- if (hctx->flags & BLK_MQ_F_BLOCKING)
- cleanup_srcu_struct(&hctx->queue_rq_srcu);
-
blk_mq_remove_cpuhp(hctx);
blk_free_flush_queue(hctx->fq);
sbitmap_free(&hctx->ctx_map);
@@ -1942,9 +1960,6 @@ static int blk_mq_init_hctx(struct request_queue *q,
node))
goto free_fq;

- if (hctx->flags & BLK_MQ_F_BLOCKING)
- init_srcu_struct(&hctx->queue_rq_srcu);
-
blk_mq_debugfs_register_hctx(q, hctx);

return 0;
@@ -2272,12 +2287,27 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
blk_mq_sysfs_register(q);
}

+static void blk_queue_dispatch_counter_release(struct percpu_ref *ref)
+{
+ struct request_queue *q =
+ container_of(ref, struct request_queue, q_dispatch_counter);
+
+ wake_up_all(&q->quiesce_wq);
+}
+
struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
struct request_queue *q)
{
/* mark the queue as mq asap */
q->mq_ops = set->ops;

+ init_waitqueue_head(&q->quiesce_wq);
+ mutex_init(&q->quiesce_mutex);
+ if (percpu_ref_init(&q->q_dispatch_counter,
+ blk_queue_dispatch_counter_release,
+ 0, GFP_KERNEL))
+ goto err_ref;
+
q->poll_cb = blk_stat_alloc_callback(blk_mq_poll_stats_fn,
blk_mq_poll_stats_bkt,
BLK_MQ_POLL_STATS_BKTS, q);
@@ -2360,6 +2390,8 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
err_percpu:
free_percpu(q->queue_ctx);
err_exit:
+ percpu_ref_exit(&q->q_dispatch_counter);
+err_ref:
q->mq_ops = NULL;
return ERR_PTR(-ENOMEM);
}
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index fcd641032f8d..6da015e6d38a 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -39,8 +39,6 @@ struct blk_mq_hw_ctx {
struct blk_mq_tags *tags;
struct blk_mq_tags *sched_tags;

- struct srcu_struct queue_rq_srcu;
-
unsigned long queued;
unsigned long run;
#define BLK_MQ_MAX_DISPATCH_ORDER 7
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 60967797f4f6..a85841e9113d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -586,6 +586,11 @@ struct request_queue {

size_t cmd_size;
void *rq_alloc_data;
+
+ /* for blk_mq_quiesce_queue() */
+ wait_queue_head_t quiesce_wq;
+ struct percpu_ref q_dispatch_counter;
+ struct mutex quiesce_mutex;
};

#define QUEUE_FLAG_QUEUED 1 /* uses generic tag queueing */
--
2.9.4