blk-mq-tag.c

  1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Tag allocation using scalable bitmaps. Uses active queue tracking to support
  4 * fairer distribution of tags between multiple submitters when a shared tag map
  5 * is used.
  6 *
  7 * Copyright (C) 2013-2014 Jens Axboe
  8 */
  9#include <linux/kernel.h>
 10#include <linux/module.h>
 11#include <linux/slab.h>
 12#include <linux/mm.h>
 13#include <linux/kmemleak.h>
 14
 15#include <linux/delay.h>
 16#include "blk.h"
 17#include "blk-mq.h"
 18#include "blk-mq-sched.h"
 19
 20/*
 21 * Recalculate wakeup batch when tag is shared by hctx.
 22 */
 23static void blk_mq_update_wake_batch(struct blk_mq_tags *tags,
 24		unsigned int users)
 25{
 26	if (!users)
 27		return;
 28
 29	sbitmap_queue_recalculate_wake_batch(&tags->bitmap_tags,
 30			users);
 31	sbitmap_queue_recalculate_wake_batch(&tags->breserved_tags,
 32			users);
 33}
 34
 35/*
 36 * If a previously inactive queue goes active, bump the active user count.
 37 * We need to do this before try to allocate driver tag, then even if fail
 38 * to get tag when first time, the other shared-tag users could reserve
 39 * budget for it.
 40 */
 41void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 42{
 43	unsigned int users;
 44	unsigned long flags;
 45	struct blk_mq_tags *tags = hctx->tags;
 46
 47	/*
 48	 * calling test_bit() prior to test_and_set_bit() is intentional,
 49	 * it avoids dirtying the cacheline if the queue is already active.
 50	 */
 51	if (blk_mq_is_shared_tags(hctx->flags)) {
 52		struct request_queue *q = hctx->queue;
 53
 54		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
 55		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
 56			return;
 57	} else {
 58		if (test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) ||
 59		    test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 60			return;
 61	}
 62
 63	spin_lock_irqsave(&tags->lock, flags);
 64	users = tags->active_queues + 1;
 65	WRITE_ONCE(tags->active_queues, users);
 66	blk_mq_update_wake_batch(tags, users);
 67	spin_unlock_irqrestore(&tags->lock, flags);
 68}
 69
 70/*
 71 * Wakeup all potentially sleeping on tags
 72 */
 73void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool include_reserve)
 74{
 75	sbitmap_queue_wake_all(&tags->bitmap_tags);
 76	if (include_reserve)
 77		sbitmap_queue_wake_all(&tags->breserved_tags);
 78}
 79
 80/*
 81 * If a previously busy queue goes inactive, potential waiters could now
 82 * be allowed to queue. Wake them up and check.
 83 */
 84void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
 85{
 86	struct blk_mq_tags *tags = hctx->tags;
 87	unsigned int users;
 88
 89	if (blk_mq_is_shared_tags(hctx->flags)) {
 90		struct request_queue *q = hctx->queue;
 91
 92		if (!test_and_clear_bit(QUEUE_FLAG_HCTX_ACTIVE,
 93					&q->queue_flags))
 94			return;
 95	} else {
 96		if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 97			return;
 98	}
 99
100	spin_lock_irq(&tags->lock);
101	users = tags->active_queues - 1;
102	WRITE_ONCE(tags->active_queues, users);
103	blk_mq_update_wake_batch(tags, users);
104	spin_unlock_irq(&tags->lock);
105
106	blk_mq_tag_wakeup_all(tags, false);
107}
108
109static int __blk_mq_get_tag(struct blk_mq_alloc_data *data,
110			    struct sbitmap_queue *bt)
111{
112	if (!data->q->elevator && !(data->flags & BLK_MQ_REQ_RESERVED) &&
113			!hctx_may_queue(data->hctx, bt))
114		return BLK_MQ_NO_TAG;
115
116	if (data->shallow_depth)
117		return sbitmap_queue_get_shallow(bt, data->shallow_depth);
118	else
119		return __sbitmap_queue_get(bt);
120}
121
122unsigned long blk_mq_get_tags(struct blk_mq_alloc_data *data, int nr_tags,
123			      unsigned int *offset)
124{
125	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
126	struct sbitmap_queue *bt = &tags->bitmap_tags;
127	unsigned long ret;
128
129	if (data->shallow_depth ||data->flags & BLK_MQ_REQ_RESERVED ||
130	    data->hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)
131		return 0;
132	ret = __sbitmap_queue_get_batch(bt, nr_tags, offset);
133	*offset += tags->nr_reserved_tags;
134	return ret;
135}
136
137unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
138{
139	struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
140	struct sbitmap_queue *bt;
141	struct sbq_wait_state *ws;
142	DEFINE_SBQ_WAIT(wait);
143	unsigned int tag_offset;
144	int tag;
145
146	if (data->flags & BLK_MQ_REQ_RESERVED) {
147		if (unlikely(!tags->nr_reserved_tags)) {
148			WARN_ON_ONCE(1);
149			return BLK_MQ_NO_TAG;
150		}
151		bt = &tags->breserved_tags;
152		tag_offset = 0;
153	} else {
154		bt = &tags->bitmap_tags;
155		tag_offset = tags->nr_reserved_tags;
156	}
157
158	tag = __blk_mq_get_tag(data, bt);
159	if (tag != BLK_MQ_NO_TAG)
160		goto found_tag;
161
162	if (data->flags & BLK_MQ_REQ_NOWAIT)
163		return BLK_MQ_NO_TAG;
164
165	ws = bt_wait_ptr(bt, data->hctx);
166	do {
167		struct sbitmap_queue *bt_prev;
168
169		/*
170		 * We're out of tags on this hardware queue, kick any
171		 * pending IO submits before going to sleep waiting for
172		 * some to complete.
173		 */
174		blk_mq_run_hw_queue(data->hctx, false);
175
176		/*
177		 * Retry tag allocation after running the hardware queue,
178		 * as running the queue may also have found completions.
179		 */
180		tag = __blk_mq_get_tag(data, bt);
181		if (tag != BLK_MQ_NO_TAG)
182			break;
183
184		sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
185
186		tag = __blk_mq_get_tag(data, bt);
187		if (tag != BLK_MQ_NO_TAG)
188			break;
189
190		bt_prev = bt;
191		io_schedule();
192
193		sbitmap_finish_wait(bt, ws, &wait);
194
195		data->ctx = blk_mq_get_ctx(data->q);
196		data->hctx = blk_mq_map_queue(data->cmd_flags, data->ctx);
197		tags = blk_mq_tags_from_data(data);
198		if (data->flags & BLK_MQ_REQ_RESERVED)
199			bt = &tags->breserved_tags;
200		else
201			bt = &tags->bitmap_tags;
202
203		/*
204		 * If destination hw queue is changed, fake wake up on
205		 * previous queue for compensating the wake up miss, so
206		 * other allocations on previous queue won't be starved.
207		 */
208		if (bt != bt_prev)
209			sbitmap_queue_wake_up(bt_prev, 1);
210
211		ws = bt_wait_ptr(bt, data->hctx);
212	} while (1);
213
214	sbitmap_finish_wait(bt, ws, &wait);
215
216found_tag:
217	/*
218	 * Give up this allocation if the hctx is inactive.  The caller will
219	 * retry on an active hctx.
220	 */
221	if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &data->hctx->state))) {
222		blk_mq_put_tag(tags, data->ctx, tag + tag_offset);
223		return BLK_MQ_NO_TAG;
224	}
225	return tag + tag_offset;
226}
227
228void blk_mq_put_tag(struct blk_mq_tags *tags, struct blk_mq_ctx *ctx,
229		    unsigned int tag)
230{
231	if (!blk_mq_tag_is_reserved(tags, tag)) {
232		const int real_tag = tag - tags->nr_reserved_tags;
233
234		BUG_ON(real_tag >= tags->nr_tags);
235		sbitmap_queue_clear(&tags->bitmap_tags, real_tag, ctx->cpu);
236	} else {
237		sbitmap_queue_clear(&tags->breserved_tags, tag, ctx->cpu);
238	}
239}
240
241void blk_mq_put_tags(struct blk_mq_tags *tags, int *tag_array, int nr_tags)
242{
243	sbitmap_queue_clear_batch(&tags->bitmap_tags, tags->nr_reserved_tags,
244					tag_array, nr_tags);
245}
246
247struct bt_iter_data {
248	struct blk_mq_hw_ctx *hctx;
249	struct request_queue *q;
250	busy_tag_iter_fn *fn;
251	void *data;
252	bool reserved;
253};
254
255static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
256		unsigned int bitnr)
257{
258	struct request *rq;
259
260	rq = tags->rqs[bitnr];
261	if (!rq || rq->tag != bitnr || !req_ref_inc_not_zero(rq))
262		rq = NULL;
263	return rq;
264}
265
266static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
267{
268	struct bt_iter_data *iter_data = data;
269	struct blk_mq_hw_ctx *hctx = iter_data->hctx;
270	struct request_queue *q = iter_data->q;
271	struct blk_mq_tag_set *set = q->tag_set;
272	struct blk_mq_tags *tags;
273	struct request *rq;
274	bool ret = true;
275
276	if (blk_mq_is_shared_tags(set->flags))
277		tags = set->shared_tags;
278	else
279		tags = hctx->tags;
280
281	if (!iter_data->reserved)
282		bitnr += tags->nr_reserved_tags;
283	/*
284	 * We can hit rq == NULL here, because the tagging functions
285	 * test and set the bit before assigning ->rqs[].
286	 */
287	rq = blk_mq_find_and_get_req(tags, bitnr);
288	if (!rq)
289		return true;
290
291	if (rq->q == q && (!hctx || rq->mq_hctx == hctx))
292		ret = iter_data->fn(rq, iter_data->data);
293	blk_mq_put_rq_ref(rq);
294	return ret;
295}
296
297/**
298 * bt_for_each - iterate over the requests associated with a hardware queue
299 * @hctx:	Hardware queue to examine.
300 * @q:		Request queue @hctx is associated with (@hctx->queue).
301 * @bt:		sbitmap to examine. This is either the breserved_tags member
302 *		or the bitmap_tags member of struct blk_mq_tags.
303 * @fn:		Pointer to the function that will be called for each request
304 *		associated with @hctx that has been assigned a driver tag.
305 *		@fn will be called as follows: @fn(rq, @data) where rq is a
306 *		pointer to a request. Return %true to continue iterating tags;
307 *		%false to stop.
308 * @data:	Will be passed as second argument to @fn.
309 * @reserved:	Indicates whether @bt is the breserved_tags member or the
310 *		bitmap_tags member of struct blk_mq_tags.
311 */
312static void bt_for_each(struct blk_mq_hw_ctx *hctx, struct request_queue *q,
313			struct sbitmap_queue *bt, busy_tag_iter_fn *fn,
314			void *data, bool reserved)
315{
316	struct bt_iter_data iter_data = {
317		.hctx = hctx,
318		.fn = fn,
319		.data = data,
320		.reserved = reserved,
321		.q = q,
322	};
323
324	sbitmap_for_each_set(&bt->sb, bt_iter, &iter_data);
325}
326
327struct bt_tags_iter_data {
328	struct blk_mq_tags *tags;
329	busy_tag_iter_fn *fn;
330	void *data;
331	unsigned int flags;
332};
333
334#define BT_TAG_ITER_RESERVED		(1 << 0)
335#define BT_TAG_ITER_STARTED		(1 << 1)
336#define BT_TAG_ITER_STATIC_RQS		(1 << 2)
337
338static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
339{
340	struct bt_tags_iter_data *iter_data = data;
341	struct blk_mq_tags *tags = iter_data->tags;
342	struct request *rq;
343	bool ret = true;
344	bool iter_static_rqs = !!(iter_data->flags & BT_TAG_ITER_STATIC_RQS);
345
346	if (!(iter_data->flags & BT_TAG_ITER_RESERVED))
347		bitnr += tags->nr_reserved_tags;
348
349	/*
350	 * We can hit rq == NULL here, because the tagging functions
351	 * test and set the bit before assigning ->rqs[].
352	 */
353	if (iter_static_rqs)
354		rq = tags->static_rqs[bitnr];
355	else
356		rq = blk_mq_find_and_get_req(tags, bitnr);
357	if (!rq)
358		return true;
359
360	if (!(iter_data->flags & BT_TAG_ITER_STARTED) ||
361	    blk_mq_request_started(rq))
362		ret = iter_data->fn(rq, iter_data->data);
363	if (!iter_static_rqs)
364		blk_mq_put_rq_ref(rq);
365	return ret;
366}
367
368/**
369 * bt_tags_for_each - iterate over the requests in a tag map
370 * @tags:	Tag map to iterate over.
371 * @bt:		sbitmap to examine. This is either the breserved_tags member
372 *		or the bitmap_tags member of struct blk_mq_tags.
373 * @fn:		Pointer to the function that will be called for each started
374 *		request. @fn will be called as follows: @fn(rq, @data) where rq
375 *		is a pointer to a request. Return %true to continue iterating
376 *		tags; %false to stop.
377 * @data:	Will be passed as second argument to @fn.
378 * @flags:	BT_TAG_ITER_*
379 */
380static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
381			     busy_tag_iter_fn *fn, void *data, unsigned int flags)
382{
383	struct bt_tags_iter_data iter_data = {
384		.tags = tags,
385		.fn = fn,
386		.data = data,
387		.flags = flags,
388	};
389
390	if (tags->rqs)
391		sbitmap_for_each_set(&bt->sb, bt_tags_iter, &iter_data);
392}
393
394static void __blk_mq_all_tag_iter(struct blk_mq_tags *tags,
395		busy_tag_iter_fn *fn, void *priv, unsigned int flags)
396{
397	WARN_ON_ONCE(flags & BT_TAG_ITER_RESERVED);
398
399	if (tags->nr_reserved_tags)
400		bt_tags_for_each(tags, &tags->breserved_tags, fn, priv,
401				 flags | BT_TAG_ITER_RESERVED);
402	bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, flags);
403}
404
405/**
406 * blk_mq_all_tag_iter - iterate over all requests in a tag map
407 * @tags:	Tag map to iterate over.
408 * @fn:		Pointer to the function that will be called for each
409 *		request. @fn will be called as follows: @fn(rq, @priv) where rq
410 *		is a pointer to a request. Return %true to continue iterating
411 *		tags; %false to stop.
412 * @priv:	Will be passed as second argument to @fn.
413 *
414 * Caller has to pass the tag map from which requests are allocated.
415 */
416void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
417		void *priv)
418{
419	__blk_mq_all_tag_iter(tags, fn, priv, BT_TAG_ITER_STATIC_RQS);
420}
421
422/**
423 * blk_mq_tagset_busy_iter - iterate over all started requests in a tag set
424 * @tagset:	Tag set to iterate over.
425 * @fn:		Pointer to the function that will be called for each started
426 *		request. @fn will be called as follows: @fn(rq, @priv) where
427 *		rq is a pointer to a request. Return true to continue iterating
428 *		tags, false to stop.
429 * @priv:	Will be passed as second argument to @fn.
430 *
431 * We grab one request reference before calling @fn and release it after
432 * @fn returns.
433 */
434void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
435		busy_tag_iter_fn *fn, void *priv)
436{
437	unsigned int flags = tagset->flags;
438	int i, nr_tags, srcu_idx;
439
440	srcu_idx = srcu_read_lock(&tagset->tags_srcu);
441
442	nr_tags = blk_mq_is_shared_tags(flags) ? 1 : tagset->nr_hw_queues;
443
444	for (i = 0; i < nr_tags; i++) {
445		if (tagset->tags && tagset->tags[i])
446			__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
447					      BT_TAG_ITER_STARTED);
448	}
449	srcu_read_unlock(&tagset->tags_srcu, srcu_idx);
450}
451EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
452
453static bool blk_mq_tagset_count_completed_rqs(struct request *rq, void *data)
454{
455	unsigned *count = data;
456
457	if (blk_mq_request_completed(rq))
458		(*count)++;
459	return true;
460}
461
462/**
463 * blk_mq_tagset_wait_completed_request - Wait until all scheduled request
464 * completions have finished.
465 * @tagset:	Tag set to drain completed request
466 *
467 * Note: This function has to be run after all IO queues are shutdown
468 */
469void blk_mq_tagset_wait_completed_request(struct blk_mq_tag_set *tagset)
470{
471	while (true) {
472		unsigned count = 0;
473
474		blk_mq_tagset_busy_iter(tagset,
475				blk_mq_tagset_count_completed_rqs, &count);
476		if (!count)
477			break;
478		msleep(5);
479	}
480}
481EXPORT_SYMBOL(blk_mq_tagset_wait_completed_request);
482
483/**
484 * blk_mq_queue_tag_busy_iter - iterate over all requests with a driver tag
485 * @q:		Request queue to examine.
486 * @fn:		Pointer to the function that will be called for each request
487 *		on @q. @fn will be called as follows: @fn(rq, @priv) where rq
488 *		is a pointer to a request and hctx points to the hardware queue
489 *		associated with the request.
490 * @priv:	Will be passed as second argument to @fn.
491 *
492 * Note: if @q->tag_set is shared with other request queues then @fn will be
493 * called for all requests on all queues that share that tag set and not only
494 * for requests associated with @q.
495 */
496void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
497		void *priv)
498{
499	int srcu_idx;
500
501	/*
502	 * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
503	 * while the queue is frozen. So we can use q_usage_counter to avoid
504	 * racing with it.
505	 */
506	if (!percpu_ref_tryget(&q->q_usage_counter))
507		return;
508
509	srcu_idx = srcu_read_lock(&q->tag_set->tags_srcu);
510	if (blk_mq_is_shared_tags(q->tag_set->flags)) {
511		struct blk_mq_tags *tags = q->tag_set->shared_tags;
512		struct sbitmap_queue *bresv = &tags->breserved_tags;
513		struct sbitmap_queue *btags = &tags->bitmap_tags;
514
515		if (tags->nr_reserved_tags)
516			bt_for_each(NULL, q, bresv, fn, priv, true);
517		bt_for_each(NULL, q, btags, fn, priv, false);
518	} else {
519		struct blk_mq_hw_ctx *hctx;
520		unsigned long i;
521
522		queue_for_each_hw_ctx(q, hctx, i) {
523			struct blk_mq_tags *tags = hctx->tags;
524			struct sbitmap_queue *bresv = &tags->breserved_tags;
525			struct sbitmap_queue *btags = &tags->bitmap_tags;
526
527			/*
528			 * If no software queues are currently mapped to this
529			 * hardware queue, there's nothing to check
530			 */
531			if (!blk_mq_hw_queue_mapped(hctx))
532				continue;
533
534			if (tags->nr_reserved_tags)
535				bt_for_each(hctx, q, bresv, fn, priv, true);
536			bt_for_each(hctx, q, btags, fn, priv, false);
537		}
538	}
539	srcu_read_unlock(&q->tag_set->tags_srcu, srcu_idx);
540	blk_queue_exit(q);
541}
542
543static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
544		    bool round_robin, int node)
545{
546	return sbitmap_queue_init_node(bt, depth, -1, round_robin, GFP_KERNEL,
547				       node);
548}
549
550struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
551		unsigned int reserved_tags, unsigned int flags, int node)
552{
553	unsigned int depth = total_tags - reserved_tags;
554	bool round_robin = flags & BLK_MQ_F_TAG_RR;
555	struct blk_mq_tags *tags;
556
557	if (total_tags > BLK_MQ_TAG_MAX) {
558		pr_err("blk-mq: tag depth too large\n");
559		return NULL;
560	}
561
562	tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
563	if (!tags)
564		return NULL;
565
566	tags->nr_tags = total_tags;
567	tags->nr_reserved_tags = reserved_tags;
568	spin_lock_init(&tags->lock);
569	INIT_LIST_HEAD(&tags->page_list);
570
571	if (bt_alloc(&tags->bitmap_tags, depth, round_robin, node))
572		goto out_free_tags;
573	if (bt_alloc(&tags->breserved_tags, reserved_tags, round_robin, node))
574		goto out_free_bitmap_tags;
575
576	return tags;
577
578out_free_bitmap_tags:
579	sbitmap_queue_free(&tags->bitmap_tags);
580out_free_tags:
581	kfree(tags);
582	return NULL;
583}
584
585static void blk_mq_free_tags_callback(struct rcu_head *head)
586{
587	struct blk_mq_tags *tags = container_of(head, struct blk_mq_tags,
588						rcu_head);
589	struct page *page;
590
591	while (!list_empty(&tags->page_list)) {
592		page = list_first_entry(&tags->page_list, struct page, lru);
593		list_del_init(&page->lru);
594		/*
595		 * Remove kmemleak object previously allocated in
596		 * blk_mq_alloc_rqs().
597		 */
598		kmemleak_free(page_address(page));
599		__free_pages(page, page->private);
600	}
601	kfree(tags);
602}
603
604void blk_mq_free_tags(struct blk_mq_tag_set *set, struct blk_mq_tags *tags)
605{
606	sbitmap_queue_free(&tags->bitmap_tags);
607	sbitmap_queue_free(&tags->breserved_tags);
608
609	/* if tags pages is not allocated yet, free tags directly */
610	if (list_empty(&tags->page_list)) {
611		kfree(tags);
612		return;
613	}
614
615	call_srcu(&set->tags_srcu, &tags->rcu_head, blk_mq_free_tags_callback);
616}
617
618void blk_mq_tag_resize_shared_tags(struct blk_mq_tag_set *set, unsigned int size)
619{
620	struct blk_mq_tags *tags = set->shared_tags;
621
622	sbitmap_queue_resize(&tags->bitmap_tags, size - set->reserved_tags);
623}
624
625void blk_mq_tag_update_sched_shared_tags(struct request_queue *q,
626					 unsigned int nr)
627{
628	sbitmap_queue_resize(&q->sched_shared_tags->bitmap_tags,
629			     nr - q->tag_set->reserved_tags);
630}
631
632/**
633 * blk_mq_unique_tag() - return a tag that is unique queue-wide
634 * @rq: request for which to compute a unique tag
635 *
636 * The tag field in struct request is unique per hardware queue but not over
637 * all hardware queues. Hence this function that returns a tag with the
638 * hardware context index in the upper bits and the per hardware queue tag in
639 * the lower bits.
640 *
641 * Note: When called for a request that is queued on a non-multiqueue request
642 * queue, the hardware context index is set to zero.
643 */
644u32 blk_mq_unique_tag(struct request *rq)
645{
646	return (rq->mq_hctx->queue_num << BLK_MQ_UNIQUE_TAG_BITS) |
647		(rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
648}
649EXPORT_SYMBOL(blk_mq_unique_tag);