[linux-2.6-block.git] / block / blk-tag.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Functions related to tagged command queuing
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/slab.h>

#include "blk.h"

/**
 * blk_queue_find_tag - find a request by its tag and queue
 * @q:	 The request queue for the device
 * @tag: The tag of the request
 *
 * Notes:
 *    Should be used when a device returns a tag and you want to match
 *    it with a request.
 *
 *    no locks need be held.
 **/
struct request *blk_queue_find_tag(struct request_queue *q, int tag)
{
	return blk_map_queue_find_tag(q->queue_tags, tag);
}
EXPORT_SYMBOL(blk_queue_find_tag);

/**
 * blk_free_tags - release a given set of tag maintenance info
 * @bqt:	the tag map to free
 *
 * Drop the reference count on @bqt and frees it when the last reference
 * is dropped.
 */
void blk_free_tags(struct blk_queue_tag *bqt)
{
	if (atomic_dec_and_test(&bqt->refcnt)) {
		BUG_ON(find_first_bit(bqt->tag_map, bqt->max_depth) <
							bqt->max_depth);

		kfree(bqt->tag_index);
		bqt->tag_index = NULL;

		kfree(bqt->tag_map);
		bqt->tag_map = NULL;

		kfree(bqt);
	}
}
EXPORT_SYMBOL(blk_free_tags);

/**
 * __blk_queue_free_tags - release tag maintenance info
 * @q:  the request queue for the device
 *
 *  Notes:
 *    blk_cleanup_queue() will take care of calling this function, if tagging
 *    has been used. So there's no need to call this directly.
 **/
void __blk_queue_free_tags(struct request_queue *q)
{
	struct blk_queue_tag *bqt = q->queue_tags;

	if (!bqt)
		return;

	blk_free_tags(bqt);

	q->queue_tags = NULL;
	queue_flag_clear_unlocked(QUEUE_FLAG_QUEUED, q);
}

/**
 * blk_queue_free_tags - release tag maintenance info
 * @q:  the request queue for the device
 *
 *  Notes:
 *	This is used to disable tagged queuing to a device, yet leave
 *	queue in function.
 **/
void blk_queue_free_tags(struct request_queue *q)
{
	queue_flag_clear_unlocked(QUEUE_FLAG_QUEUED, q);
}
EXPORT_SYMBOL(blk_queue_free_tags);

static int
init_tag_map(struct request_queue *q, struct blk_queue_tag *tags, int depth)
{
	struct request **tag_index;
	unsigned long *tag_map;
	int nr_ulongs;

	if (q && depth > q->nr_requests * 2) {
		depth = q->nr_requests * 2;
		printk(KERN_ERR "%s: adjusted depth to %d\n",
		       __func__, depth);
	}

	tag_index = kcalloc(depth, sizeof(struct request *), GFP_ATOMIC);
	if (!tag_index)
		goto fail;

	nr_ulongs = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;
	tag_map = kcalloc(nr_ulongs, sizeof(unsigned long), GFP_ATOMIC);
	if (!tag_map)
		goto fail;

	tags->real_max_depth = depth;
	tags->max_depth = depth;
	tags->tag_index = tag_index;
	tags->tag_map = tag_map;

	return 0;
fail:
	kfree(tag_index);
	return -ENOMEM;
}

static struct blk_queue_tag *__blk_queue_init_tags(struct request_queue *q,
						int depth, int alloc_policy)
{
	struct blk_queue_tag *tags;

	tags = kmalloc(sizeof(struct blk_queue_tag), GFP_ATOMIC);
	if (!tags)
		goto fail;

	if (init_tag_map(q, tags, depth))
		goto fail;

	atomic_set(&tags->refcnt, 1);
	tags->alloc_policy = alloc_policy;
	tags->next_tag = 0;
	return tags;
fail:
	kfree(tags);
	return NULL;
}

/**
 * blk_init_tags - initialize the tag info for an external tag map
 * @depth:	the maximum queue depth supported
 * @alloc_policy: tag allocation policy
 **/
struct blk_queue_tag *blk_init_tags(int depth, int alloc_policy)
{
	return __blk_queue_init_tags(NULL, depth, alloc_policy);
}
EXPORT_SYMBOL(blk_init_tags);

/**
 * blk_queue_init_tags - initialize the queue tag info
 * @q:  the request queue for the device
 * @depth:  the maximum queue depth supported
 * @tags: the tag to use
 * @alloc_policy: tag allocation policy
 *
 * Queue lock must be held here if the function is called to resize an
 * existing map.
 **/
int blk_queue_init_tags(struct request_queue *q, int depth,
			struct blk_queue_tag *tags, int alloc_policy)
{
	int rc;

	BUG_ON(tags && q->queue_tags && tags != q->queue_tags);

	if (!tags && !q->queue_tags) {
		tags = __blk_queue_init_tags(q, depth, alloc_policy);

		if (!tags)
			return -ENOMEM;

	} else if (q->queue_tags) {
		rc = blk_queue_resize_tags(q, depth);
		if (rc)
			return rc;
		queue_flag_set(QUEUE_FLAG_QUEUED, q);
		return 0;
	} else
		atomic_inc(&tags->refcnt);

	/*
	 * assign it, all done
	 */
	q->queue_tags = tags;
	queue_flag_set_unlocked(QUEUE_FLAG_QUEUED, q);
	return 0;
}
EXPORT_SYMBOL(blk_queue_init_tags);

/**
 * blk_queue_resize_tags - change the queueing depth
 * @q:  the request queue for the device
 * @new_depth: the new max command queueing depth
 *
 *  Notes:
 *    Must be called with the queue lock held.
 **/
int blk_queue_resize_tags(struct request_queue *q, int new_depth)
{
	struct blk_queue_tag *bqt = q->queue_tags;
	struct request **tag_index;
	unsigned long *tag_map;
	int max_depth, nr_ulongs;

	if (!bqt)
		return -ENXIO;

	/*
	 * if we already have large enough real_max_depth.  just
	 * adjust max_depth.  *NOTE* as requests with tag value
	 * between new_depth and real_max_depth can be in-flight, tag
	 * map can not be shrunk blindly here.
	 */
	if (new_depth <= bqt->real_max_depth) {
		bqt->max_depth = new_depth;
		return 0;
	}

	/*
	 * Currently cannot replace a shared tag map with a new
	 * one, so error out if this is the case
	 */
	if (atomic_read(&bqt->refcnt) != 1)
		return -EBUSY;

	/*
	 * save the old state info, so we can copy it back
	 */
	tag_index = bqt->tag_index;
	tag_map = bqt->tag_map;
	max_depth = bqt->real_max_depth;

	if (init_tag_map(q, bqt, new_depth))
		return -ENOMEM;

	memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
	nr_ulongs = ALIGN(max_depth, BITS_PER_LONG) / BITS_PER_LONG;
	memcpy(bqt->tag_map, tag_map, nr_ulongs * sizeof(unsigned long));

	kfree(tag_index);
	kfree(tag_map);
	return 0;
}
EXPORT_SYMBOL(blk_queue_resize_tags);

/**
 * blk_queue_end_tag - end tag operations for a request
 * @q:  the request queue for the device
 * @rq: the request that has completed
 *
 *  Description:
 *    Typically called when end_that_request_first() returns %0, meaning
 *    all transfers have been done for a request. It's important to call
 *    this function before end_that_request_last(), as that will put the
 *    request back on the free list thus corrupting the internal tag list.
 **/
void blk_queue_end_tag(struct request_queue *q, struct request *rq)
{
	struct blk_queue_tag *bqt = q->queue_tags;
	unsigned tag = rq->tag; /* negative tags invalid */

	lockdep_assert_held(q->queue_lock);

	BUG_ON(tag >= bqt->real_max_depth);

	list_del_init(&rq->queuelist);
	rq->rq_flags &= ~RQF_QUEUED;
	rq->tag = -1;
	rq->internal_tag = -1;

	if (unlikely(bqt->tag_index[tag] == NULL))
		printk(KERN_ERR "%s: tag %d is missing\n",
		       __func__, tag);

	bqt->tag_index[tag] = NULL;

	if (unlikely(!test_bit(tag, bqt->tag_map))) {
		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
		       __func__, tag);
		return;
	}
	/*
	 * The tag_map bit acts as a lock for tag_index[bit], so we need
	 * unlock memory barrier semantics.
	 */
	clear_bit_unlock(tag, bqt->tag_map);
}

/**
 * blk_queue_start_tag - find a free tag and assign it
 * @q:  the request queue for the device
 * @rq:  the block request that needs tagging
 *
 *  Description:
 *    This can either be used as a stand-alone helper, or possibly be
 *    assigned as the queue &prep_rq_fn (in which case &struct request
 *    automagically gets a tag assigned). Note that this function
 *    assumes that any type of request can be queued! if this is not
 *    true for your device, you must check the request type before
 *    calling this function.  The request will also be removed from
 *    the request queue, so it's the drivers responsibility to readd
 *    it if it should need to be restarted for some reason.
 **/
int blk_queue_start_tag(struct request_queue *q, struct request *rq)
{
	struct blk_queue_tag *bqt = q->queue_tags;
	unsigned max_depth;
	int tag;

	lockdep_assert_held(q->queue_lock);

	if (unlikely((rq->rq_flags & RQF_QUEUED))) {
		printk(KERN_ERR
		       "%s: request %p for device [%s] already tagged %d",
		       __func__, rq,
		       rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->tag);
		BUG();
	}

	/*
	 * Protect against shared tag maps, as we may not have exclusive
	 * access to the tag map.
	 *
	 * We reserve a few tags just for sync IO, since we don't want
	 * to starve sync IO on behalf of flooding async IO.
	 */
	max_depth = bqt->max_depth;
	if (!rq_is_sync(rq) && max_depth > 1) {
		switch (max_depth) {
		case 2:
			max_depth = 1;
			break;
		case 3:
			max_depth = 2;
			break;
		default:
			max_depth -= 2;
		}
		if (q->in_flight[BLK_RW_ASYNC] > max_depth)
			return 1;
	}

	do {
		if (bqt->alloc_policy == BLK_TAG_ALLOC_FIFO) {
			tag = find_first_zero_bit(bqt->tag_map, max_depth);
			if (tag >= max_depth)
				return 1;
		} else {
			int start = bqt->next_tag;
			int size = min_t(int, bqt->max_depth, max_depth + start);
			tag = find_next_zero_bit(bqt->tag_map, size, start);
			if (tag >= size && start + size > bqt->max_depth) {
				size = start + size - bqt->max_depth;
				tag = find_first_zero_bit(bqt->tag_map, size);
			}
			if (tag >= size)
				return 1;
		}

	} while (test_and_set_bit_lock(tag, bqt->tag_map));
	/*
	 * We need lock ordering semantics given by test_and_set_bit_lock.
	 * See blk_queue_end_tag for details.
	 */

	bqt->next_tag = (tag + 1) % bqt->max_depth;
	rq->rq_flags |= RQF_QUEUED;
	rq->tag = tag;
	bqt->tag_index[tag] = rq;
	blk_start_request(rq);
	return 0;
}
EXPORT_SYMBOL(blk_queue_start_tag);
Commit	Line	Data
b2441318	1	// SPDX-License-Identifier: GPL-2.0
8324aa91 JA	2	/*
	3	* Functions related to tagged command queuing
	4	*/
	5	#include <linux/kernel.h>
	6	#include <linux/module.h>
	7	#include <linux/bio.h>
	8	#include <linux/blkdev.h>
5a0e3ad6	9	#include <linux/slab.h>
8324aa91	10
278caf01 AB	11	#include "blk.h"
278caf01 AB	12
8324aa91 JA	13	/**
	14	* blk_queue_find_tag - find a request by its tag and queue
	15	* @q: The request queue for the device
	16	* @tag: The tag of the request
	17	*
	18	* Notes:
	19	* Should be used when a device returns a tag and you want to match
	20	* it with a request.
	21	*
	22	* no locks need be held.
	23	**/
	24	struct request blk_queue_find_tag(struct request_queue q, int tag)
	25	{
	26	return blk_map_queue_find_tag(q->queue_tags, tag);
	27	}
8324aa91 JA	28	EXPORT_SYMBOL(blk_queue_find_tag);
	29
	30	/**
d45b3279	31	* blk_free_tags - release a given set of tag maintenance info
8324aa91 JA	32	* @bqt: the tag map to free
8324aa91 JA	33	*
d45b3279 CH	34	* Drop the reference count on @bqt and frees it when the last reference
d45b3279 CH	35	* is dropped.
8324aa91	36	*/
d45b3279	37	void blk_free_tags(struct blk_queue_tag *bqt)
8324aa91	38	{
d45b3279	39	if (atomic_dec_and_test(&bqt->refcnt)) {
0e3eb45e MW	40	BUG_ON(find_first_bit(bqt->tag_map, bqt->max_depth) <
0e3eb45e MW	41	bqt->max_depth);
8324aa91 JA	42
	43	kfree(bqt->tag_index);
	44	bqt->tag_index = NULL;
	45
	46	kfree(bqt->tag_map);
	47	bqt->tag_map = NULL;
	48
	49	kfree(bqt);
	50	}
8324aa91	51	}
d45b3279	52	EXPORT_SYMBOL(blk_free_tags);
8324aa91 JA	53
	54	/**
	55	* __blk_queue_free_tags - release tag maintenance info
	56	* @q: the request queue for the device
	57	*
	58	* Notes:
	59	* blk_cleanup_queue() will take care of calling this function, if tagging
	60	* has been used. So there's no need to call this directly.
	61	**/
	62	void __blk_queue_free_tags(struct request_queue *q)
	63	{
	64	struct blk_queue_tag *bqt = q->queue_tags;
	65
	66	if (!bqt)
	67	return;
	68
d45b3279	69	blk_free_tags(bqt);
8324aa91 JA	70
8324aa91 JA	71	q->queue_tags = NULL;
aa94b537	72	queue_flag_clear_unlocked(QUEUE_FLAG_QUEUED, q);
8324aa91 JA	73	}
8324aa91 JA	74
8324aa91 JA	75	/**
	76	* blk_queue_free_tags - release tag maintenance info
	77	* @q: the request queue for the device
	78	*
	79	* Notes:
710027a4	80	* This is used to disable tagged queuing to a device, yet leave
8324aa91 JA	81	* queue in function.
	82	**/
	83	void blk_queue_free_tags(struct request_queue *q)
	84	{
aa94b537	85	queue_flag_clear_unlocked(QUEUE_FLAG_QUEUED, q);
8324aa91	86	}
8324aa91 JA	87	EXPORT_SYMBOL(blk_queue_free_tags);
	88
	89	static int
	90	init_tag_map(struct request_queue q, struct blk_queue_tag tags, int depth)
	91	{
	92	struct request **tag_index;
	93	unsigned long *tag_map;
	94	int nr_ulongs;
	95
	96	if (q && depth > q->nr_requests * 2) {
	97	depth = q->nr_requests * 2;
	98	printk(KERN_ERR "%s: adjusted depth to %d\n",
24c03d47	99	__func__, depth);
8324aa91 JA	100	}
8324aa91 JA	101
6396bb22	102	tag_index = kcalloc(depth, sizeof(struct request *), GFP_ATOMIC);
8324aa91 JA	103	if (!tag_index)
	104	goto fail;
	105
	106	nr_ulongs = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;
6396bb22	107	tag_map = kcalloc(nr_ulongs, sizeof(unsigned long), GFP_ATOMIC);
8324aa91 JA	108	if (!tag_map)
	109	goto fail;
	110
	111	tags->real_max_depth = depth;
	112	tags->max_depth = depth;
	113	tags->tag_index = tag_index;
	114	tags->tag_map = tag_map;
	115
	116	return 0;
	117	fail:
	118	kfree(tag_index);
	119	return -ENOMEM;
	120	}
	121
	122	static struct blk_queue_tag __blk_queue_init_tags(struct request_queue q,
ee1b6f7a	123	int depth, int alloc_policy)
8324aa91 JA	124	{
	125	struct blk_queue_tag *tags;
	126
	127	tags = kmalloc(sizeof(struct blk_queue_tag), GFP_ATOMIC);
	128	if (!tags)
	129	goto fail;
	130
	131	if (init_tag_map(q, tags, depth))
	132	goto fail;
	133
8324aa91	134	atomic_set(&tags->refcnt, 1);
ee1b6f7a SL	135	tags->alloc_policy = alloc_policy;
ee1b6f7a SL	136	tags->next_tag = 0;
8324aa91 JA	137	return tags;
	138	fail:
	139	kfree(tags);
	140	return NULL;
	141	}
	142
	143	/**
	144	* blk_init_tags - initialize the tag info for an external tag map
	145	* @depth: the maximum queue depth supported
ee1b6f7a	146	* @alloc_policy: tag allocation policy
8324aa91	147	**/
ee1b6f7a	148	struct blk_queue_tag *blk_init_tags(int depth, int alloc_policy)
8324aa91	149	{
ee1b6f7a	150	return __blk_queue_init_tags(NULL, depth, alloc_policy);
8324aa91 JA	151	}
	152	EXPORT_SYMBOL(blk_init_tags);
	153
	154	/**
	155	* blk_queue_init_tags - initialize the queue tag info
	156	* @q: the request queue for the device
	157	* @depth: the maximum queue depth supported
	158	* @tags: the tag to use
ee1b6f7a	159	* @alloc_policy: tag allocation policy
aa94b537 JA	160	*
	161	* Queue lock must be held here if the function is called to resize an
	162	* existing map.
8324aa91 JA	163	**/
8324aa91 JA	164	int blk_queue_init_tags(struct request_queue *q, int depth,
ee1b6f7a	165	struct blk_queue_tag *tags, int alloc_policy)
8324aa91 JA	166	{
	167	int rc;
	168
	169	BUG_ON(tags && q->queue_tags && tags != q->queue_tags);
	170
	171	if (!tags && !q->queue_tags) {
ee1b6f7a	172	tags = __blk_queue_init_tags(q, depth, alloc_policy);
8324aa91 JA	173
8324aa91 JA	174	if (!tags)
d41570b7 PST	175	return -ENOMEM;
d41570b7 PST	176
8324aa91	177	} else if (q->queue_tags) {
6728cb0e JA	178	rc = blk_queue_resize_tags(q, depth);
6728cb0e JA	179	if (rc)
8324aa91	180	return rc;
75ad23bc	181	queue_flag_set(QUEUE_FLAG_QUEUED, q);
8324aa91 JA	182	return 0;
	183	} else
	184	atomic_inc(&tags->refcnt);
	185
	186	/*
	187	* assign it, all done
	188	*/
	189	q->queue_tags = tags;
aa94b537	190	queue_flag_set_unlocked(QUEUE_FLAG_QUEUED, q);
8324aa91	191	return 0;
8324aa91	192	}
8324aa91 JA	193	EXPORT_SYMBOL(blk_queue_init_tags);
	194
	195	/**
	196	* blk_queue_resize_tags - change the queueing depth
	197	* @q: the request queue for the device
	198	* @new_depth: the new max command queueing depth
	199	*
	200	* Notes:
	201	* Must be called with the queue lock held.
	202	**/
	203	int blk_queue_resize_tags(struct request_queue *q, int new_depth)
	204	{
	205	struct blk_queue_tag *bqt = q->queue_tags;
	206	struct request **tag_index;
	207	unsigned long *tag_map;
	208	int max_depth, nr_ulongs;
	209
	210	if (!bqt)
	211	return -ENXIO;
	212
	213	/*
	214	* if we already have large enough real_max_depth. just
	215	* adjust max_depth. NOTE as requests with tag value
	216	* between new_depth and real_max_depth can be in-flight, tag
	217	* map can not be shrunk blindly here.
	218	*/
	219	if (new_depth <= bqt->real_max_depth) {
	220	bqt->max_depth = new_depth;
	221	return 0;
	222	}
	223
	224	/*
	225	* Currently cannot replace a shared tag map with a new
	226	* one, so error out if this is the case
	227	*/
	228	if (atomic_read(&bqt->refcnt) != 1)
	229	return -EBUSY;
	230
	231	/*
	232	* save the old state info, so we can copy it back
	233	*/
	234	tag_index = bqt->tag_index;
	235	tag_map = bqt->tag_map;
	236	max_depth = bqt->real_max_depth;
	237
	238	if (init_tag_map(q, bqt, new_depth))
	239	return -ENOMEM;
	240
	241	memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
	242	nr_ulongs = ALIGN(max_depth, BITS_PER_LONG) / BITS_PER_LONG;
	243	memcpy(bqt->tag_map, tag_map, nr_ulongs * sizeof(unsigned long));
	244
	245	kfree(tag_index);
	246	kfree(tag_map);
	247	return 0;
	248	}
8324aa91 JA	249	EXPORT_SYMBOL(blk_queue_resize_tags);
	250
	251	/**
	252	* blk_queue_end_tag - end tag operations for a request
	253	* @q: the request queue for the device
	254	* @rq: the request that has completed
	255	*
	256	* Description:
710027a4	257	* Typically called when end_that_request_first() returns %0, meaning
8324aa91 JA	258	* all transfers have been done for a request. It's important to call
	259	* this function before end_that_request_last(), as that will put the
	260	* request back on the free list thus corrupting the internal tag list.
8324aa91 JA	261	**/
	262	void blk_queue_end_tag(struct request_queue q, struct request rq)
	263	{
	264	struct blk_queue_tag *bqt = q->queue_tags;
f2b20d43	265	unsigned tag = rq->tag; /* negative tags invalid */
8324aa91	266
2fff8a92 BVA	267	lockdep_assert_held(q->queue_lock);
2fff8a92 BVA	268
f2b20d43	269	BUG_ON(tag >= bqt->real_max_depth);
8324aa91 JA	270
8324aa91 JA	271	list_del_init(&rq->queuelist);
e8064021	272	rq->rq_flags &= ~RQF_QUEUED;
8324aa91	273	rq->tag = -1;
bd166ef1	274	rq->internal_tag = -1;
8324aa91 JA	275
	276	if (unlikely(bqt->tag_index[tag] == NULL))
	277	printk(KERN_ERR "%s: tag %d is missing\n",
24c03d47	278	__func__, tag);
8324aa91 JA	279
	280	bqt->tag_index[tag] = NULL;
	281
	282	if (unlikely(!test_bit(tag, bqt->tag_map))) {
	283	printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
24c03d47	284	__func__, tag);
8324aa91 JA	285	return;
	286	}
	287	/*
	288	* The tag_map bit acts as a lock for tag_index[bit], so we need
	289	* unlock memory barrier semantics.
	290	*/
	291	clear_bit_unlock(tag, bqt->tag_map);
8324aa91	292	}
8324aa91 JA	293
	294	/**
	295	* blk_queue_start_tag - find a free tag and assign it
	296	* @q: the request queue for the device
	297	* @rq: the block request that needs tagging
	298	*
	299	* Description:
	300	* This can either be used as a stand-alone helper, or possibly be
	301	* assigned as the queue &prep_rq_fn (in which case &struct request
	302	* automagically gets a tag assigned). Note that this function
	303	* assumes that any type of request can be queued! if this is not
	304	* true for your device, you must check the request type before
	305	* calling this function. The request will also be removed from
	306	* the request queue, so it's the drivers responsibility to readd
	307	* it if it should need to be restarted for some reason.
8324aa91 JA	308	**/
	309	int blk_queue_start_tag(struct request_queue q, struct request rq)
	310	{
	311	struct blk_queue_tag *bqt = q->queue_tags;
0a7ae2ff	312	unsigned max_depth;
8324aa91 JA	313	int tag;
8324aa91 JA	314
2fff8a92 BVA	315	lockdep_assert_held(q->queue_lock);
2fff8a92 BVA	316
e8064021	317	if (unlikely((rq->rq_flags & RQF_QUEUED))) {
6728cb0e	318	printk(KERN_ERR
8324aa91	319	"%s: request %p for device [%s] already tagged %d",
24c03d47	320	__func__, rq,
8324aa91 JA	321	rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->tag);
	322	BUG();
	323	}
	324
	325	/*
	326	* Protect against shared tag maps, as we may not have exclusive
	327	* access to the tag map.
e3ba9ae5 JA	328	*
	329	* We reserve a few tags just for sync IO, since we don't want
	330	* to starve sync IO on behalf of flooding async IO.
8324aa91	331	*/
e3ba9ae5	332	max_depth = bqt->max_depth;
0a7ae2ff	333	if (!rq_is_sync(rq) && max_depth > 1) {
a6b3f761 JK	334	switch (max_depth) {
a6b3f761 JK	335	case 2:
0a7ae2ff	336	max_depth = 1;
a6b3f761 JK	337	break;
	338	case 3:
	339	max_depth = 2;
	340	break;
	341	default:
	342	max_depth -= 2;
	343	}
1b59dd51	344	if (q->in_flight[BLK_RW_ASYNC] > max_depth)
0a7ae2ff JA	345	return 1;
0a7ae2ff JA	346	}
e3ba9ae5	347
8324aa91	348	do {
ee1b6f7a SL	349	if (bqt->alloc_policy == BLK_TAG_ALLOC_FIFO) {
	350	tag = find_first_zero_bit(bqt->tag_map, max_depth);
	351	if (tag >= max_depth)
	352	return 1;
	353	} else {
	354	int start = bqt->next_tag;
	355	int size = min_t(int, bqt->max_depth, max_depth + start);
	356	tag = find_next_zero_bit(bqt->tag_map, size, start);
	357	if (tag >= size && start + size > bqt->max_depth) {
	358	size = start + size - bqt->max_depth;
	359	tag = find_first_zero_bit(bqt->tag_map, size);
	360	}
	361	if (tag >= size)
	362	return 1;
	363	}
8324aa91 JA	364
	365	} while (test_and_set_bit_lock(tag, bqt->tag_map));
	366	/*
	367	* We need lock ordering semantics given by test_and_set_bit_lock.
	368	* See blk_queue_end_tag for details.
	369	*/
	370
ee1b6f7a	371	bqt->next_tag = (tag + 1) % bqt->max_depth;
e8064021	372	rq->rq_flags \|= RQF_QUEUED;
8324aa91 JA	373	rq->tag = tag;
8324aa91 JA	374	bqt->tag_index[tag] = rq;
9934c8c0	375	blk_start_request(rq);
8324aa91 JA	376	return 0;
8324aa91 JA	377	}
8324aa91	378	EXPORT_SYMBOL(blk_queue_start_tag);