[PATCH V2 2/8] blk-mq: use static_rqs instead of rqs to iterate tags

Jianchao Wang
2019-03-25T05:47:41Z

tags->rqs[] will not been cleaned when free driver tag to avoid
an extra store on a shared area in the per io path. But there
is a window between get driver tag and write tags->rqs[], so we
may see stale rq in tags->rqs[] which may have been freed, as
following case,
blk_mq_get_request blk_mq_queue_tag_busy_iter
-> blk_mq_get_tag
-> bt_for_each
-> bt_iter
-> rq = taags->rqs[]
-> rq->q
-> blk_mq_rq_ctx_init
-> data->hctx->tags->rqs[rq->tag] = rq;

In additiion, tags->rqs[] only contains the requests that get
driver tag. It is not accurate for io-scheduler case when account
busy tags in part_in_flight.

To fix both of them, the blk_mq_queue_tag_busy_iter is changed
in this patch to use tags->static_rqs[] instead of tags->rqs[].
We have to identify whether there is a io scheduler attached to
decide to use hctx->tags or hctx->sched_tags. And we will try to
get a non-zero q_usage_counter before that, then could avoid race
with update nr_hw_queues, switch io-scheduler and even queue cleanup.

Add 'inflight' parameter to determine to iterate in-flight
requests or just busy tags and add a new helper interface
blk_mq_queue_tag_inflight_iter to iterate all of the in-flight
tags and export this interface for drivers.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
block/blk-mq-tag.c | 96 ++++++++++++++++++++++++++++++++++++--------------
block/blk-mq-tag.h | 4 +--
block/blk-mq.c | 27 +++++---------
include/linux/blk-mq.h | 5 +--
4 files changed, 83 insertions(+), 49 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 5f28002..bf7b235 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -216,26 +216,38 @@ struct bt_iter_data {
busy_iter_fn *fn;
void *data;
bool reserved;
+ bool inflight;
};

static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
{
struct bt_iter_data *iter_data = data;
struct blk_mq_hw_ctx *hctx = iter_data->hctx;
- struct blk_mq_tags *tags = hctx->tags;
bool reserved = iter_data->reserved;
+ struct blk_mq_tags *tags;
struct request *rq;

+ tags = hctx->sched_tags ? hctx->sched_tags : hctx->tags;
+
if (!reserved)
bitnr += tags->nr_reserved_tags;
- rq = tags->rqs[bitnr];
+ /*
+ * Because tags->rqs[] will not been cleaned when free driver tag
+ * and there is a window between get driver tag and write tags->rqs[],
+ * so we may see stale rq in tags->rqs[] which may have been freed.
+ * Using static_rqs[] is safer.
+ */
+ rq = tags->static_rqs[bitnr];

/*
- * We can hit rq == NULL here, because the tagging functions
- * test and set the bit before assigning ->rqs[].
+ * There is a small window between get tag and blk_mq_rq_ctx_init,
+ * so rq->q and rq->mq_hctx maybe different.
*/
- if (rq && rq->q == hctx->queue)
- return iter_data->fn(hctx, rq, iter_data->data, reserved);
+ if (rq && rq->q == hctx->queue &&
+ rq->mq_hctx == hctx &&
+ (!iter_data->inflight ||
+ blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT))
+ return iter_data->fn(rq, iter_data->data, reserved);
return true;
}

@@ -246,7 +258,7 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
* or the bitmap_tags member of struct blk_mq_tags.
* @fn: Pointer to the function that will be called for each request
* associated with @hctx that has been assigned a driver tag.
- * @fn will be called as follows: @fn(@hctx, rq, @data, @reserved)
+ * @fn will be called as follows: @fn(rq, @data, @reserved)
* where rq is a pointer to a request. Return true to continue
* iterating tags, false to stop.
* @data: Will be passed as third argument to @fn.
@@ -254,13 +266,14 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
* bitmap_tags member of struct blk_mq_tags.
*/
static void bt_for_each(struct blk_mq_hw_ctx *hctx, struct sbitmap_queue *bt,
- busy_iter_fn *fn, void *data, bool reserved)
+ busy_iter_fn *fn, void *data, bool reserved, bool inflight)
{
struct bt_iter_data iter_data = {
.hctx = hctx,
.fn = fn,
.data = data,
.reserved = reserved,
+ .inflight = inflight,
};

sbitmap_for_each_set(&bt->sb, bt_iter, &iter_data);
@@ -362,36 +375,33 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
EXPORT_SYMBOL(blk_mq_tagset_busy_iter);

/**
- * blk_mq_queue_tag_busy_iter - iterate over all requests with a driver tag
+ * __blk_mq_queue_tag_busy_iter - iterate over all busy or inflight tags
* @q: Request queue to examine.
- * @fn: Pointer to the function that will be called for each request
- * on @q. @fn will be called as follows: @fn(hctx, rq, @priv,
- * reserved) where rq is a pointer to a request and hctx points
- * to the hardware queue associated with the request. 'reserved'
- * indicates whether or not @rq is a reserved request.
+ * @fn: Pointer to the function that will be called for each
+ * in-flight request issued by @q. @fn will be called as
+ * follows:
+ * @fn(rq, @priv, reserved)
+ * rq is a pointer to a request.'reserved' indicates whether or
+ * not @rq is a reserved request.
* @priv: Will be passed as third argument to @fn.
- *
- * Note: if @q->tag_set is shared with other request queues then @fn will be
- * called for all requests on all queues that share that tag set and not only
- * for requests associated with @q.
*/
-void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
- void *priv)
+static void __blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
+ void *priv, bool inflight)
{
struct blk_mq_hw_ctx *hctx;
int i;

/*
- * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
- * while the queue is frozen. So we can use q_usage_counter to avoid
- * racing with it.
+ * Get a reference of the queue unless it has been zero. We use this
+ * to avoid the race with the code that would modify the hctxs after
+ * freeze and drain the queue, including updating nr_hw_queues, io
+ * scheduler switching and queue clean up.
*/
if (!percpu_ref_tryget(&q->q_usage_counter))
return;

queue_for_each_hw_ctx(q, hctx, i) {
- struct blk_mq_tags *tags = hctx->tags;
-
+ struct blk_mq_tags *tags;
/*
* If no software queues are currently mapped to this
* hardware queue, there's nothing to check
@@ -399,13 +409,45 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
if (!blk_mq_hw_queue_mapped(hctx))
continue;

+ tags = hctx->sched_tags ? hctx->sched_tags : hctx->tags;
+
if (tags->nr_reserved_tags)
- bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
- bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
+ bt_for_each(hctx, &tags->breserved_tags,
+ fn, priv, true, inflight);
+ bt_for_each(hctx, &tags->bitmap_tags,
+ fn, priv, false, inflight);
+ /*
+ * flush_rq represents the rq with REQ_PREFLUSH and REQ_FUA
+ * (if FUA is not supported by device) to be issued to
+ * device. So we need to consider it when iterate inflight
+ * rqs, but needn't to count it when iterate busy tags.
+ */
+ if (inflight &&
+ blk_mq_rq_state(hctx->fq->flush_rq) == MQ_RQ_IN_FLIGHT)
+ fn(hctx->fq->flush_rq, priv, false);
}
blk_queue_exit(q);
}

+/*
+ * Iterate all the busy tags including pending and in-flight ones.
+ */
+void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
+ void *priv)
+{
+ __blk_mq_queue_tag_busy_iter(q, fn, priv, false);
+}
+
+/*
+ * Iterate all the inflight tags.
+ */
+void blk_mq_queue_tag_inflight_iter(struct request_queue *q,
+ busy_iter_fn *fn, void *priv)
+{
+ __blk_mq_queue_tag_busy_iter(q, fn, priv, true);
+}
+EXPORT_SYMBOL(blk_mq_queue_tag_inflight_iter);
+
static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
bool round_robin, int node)
{
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 61deab0..bcbb699 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -32,9 +32,9 @@ extern bool blk_mq_has_free_tags(struct blk_mq_tags *tags);
extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
struct blk_mq_tags **tags,
unsigned int depth, bool can_grow);
-extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
-void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
+extern void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
void *priv);
+extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);

static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt,
struct blk_mq_hw_ctx *hctx)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2102d91..531522e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -93,8 +93,7 @@ struct mq_inflight {
unsigned int *inflight;
};

-static bool blk_mq_check_inflight(struct blk_mq_hw_ctx *hctx,
- struct request *rq, void *priv,
+static bool blk_mq_check_inflight(struct request *rq, void *priv,
bool reserved)
{
struct mq_inflight *mi = priv;
@@ -119,8 +118,7 @@ unsigned int blk_mq_in_flight(struct request_queue *q, struct hd_struct *part)
return inflight[0];
}

-static bool blk_mq_check_inflight_rw(struct blk_mq_hw_ctx *hctx,
- struct request *rq, void *priv,
+static bool blk_mq_check_inflight_rw(struct request *rq, void *priv,
bool reserved)
{
struct mq_inflight *mi = priv;
@@ -809,28 +807,22 @@ struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
}
EXPORT_SYMBOL(blk_mq_tag_to_rq);

-static bool blk_mq_rq_inflight(struct blk_mq_hw_ctx *hctx, struct request *rq,
- void *priv, bool reserved)
+static bool blk_mq_rq_inflight(struct request *rq, void *priv, bool reserved)
{
+ bool *busy = priv;
/*
* If we find a request that is inflight and the queue matches,
* we know the queue is busy. Return false to stop the iteration.
*/
- if (rq->state == MQ_RQ_IN_FLIGHT && rq->q == hctx->queue) {
- bool *busy = priv;
-
- *busy = true;
- return false;
- }
-
- return true;
+ *busy = true;
+ return false;
}

bool blk_mq_queue_inflight(struct request_queue *q)
{
bool busy = false;

- blk_mq_queue_tag_busy_iter(q, blk_mq_rq_inflight, &busy);
+ blk_mq_queue_tag_inflight_iter(q, blk_mq_rq_inflight, &busy);
return busy;
}
EXPORT_SYMBOL_GPL(blk_mq_queue_inflight);
@@ -870,8 +862,7 @@ static bool blk_mq_req_expired(struct request *rq, unsigned long *next)
return false;
}

-static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
- struct request *rq, void *priv, bool reserved)
+static bool blk_mq_check_expired(struct request *rq, void *priv, bool reserved)
{
unsigned long *next = priv;

@@ -932,7 +923,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
if (!percpu_ref_tryget(&q->q_usage_counter))
return;

- blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
+ blk_mq_queue_tag_inflight_iter(q, blk_mq_check_expired, &next);

if (next != 0) {
mod_timer(&q->timeout, next);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index b0c814b..dff9bb6 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -132,8 +132,7 @@ typedef int (init_request_fn)(struct blk_mq_tag_set *set, struct request *,
typedef void (exit_request_fn)(struct blk_mq_tag_set *set, struct request *,
unsigned int);

-typedef bool (busy_iter_fn)(struct blk_mq_hw_ctx *, struct request *, void *,
- bool);
+typedef bool (busy_iter_fn)(struct request *, void *, bool);
typedef bool (busy_tag_iter_fn)(struct request *, void *, bool);
typedef int (poll_fn)(struct blk_mq_hw_ctx *);
typedef int (map_queues_fn)(struct blk_mq_tag_set *set);
@@ -321,6 +320,8 @@ bool blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async);
void blk_mq_run_hw_queues(struct request_queue *q, bool async);
void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
busy_tag_iter_fn *fn, void *priv);
+void blk_mq_queue_tag_inflight_iter(struct request_queue *q,
+ busy_iter_fn *fn, void *priv);
void blk_mq_freeze_queue(struct request_queue *q);
void blk_mq_unfreeze_queue(struct request_queue *q);
void blk_freeze_queue_start(struct request_queue *q);
--
2.7.4