block/blk-flush.c - linux - Git at Google

 /*
  * Functions to sequence FLUSH and FUA writes.
  *
  * Copyright (C) 2011		Max Planck Institute for Gravitational Physics
  * Copyright (C) 2011		Tejun Heo <tj@kernel.org>
  *
  * This file is released under the GPLv2.
  *
  * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
  * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
  * properties and hardware capability.
  *
  * If a request doesn't have data, only REQ_FLUSH makes sense, which
  * indicates a simple flush request.  If there is data, REQ_FLUSH indicates
  * that the device cache should be flushed before the data is executed, and
  * REQ_FUA means that the data must be on non-volatile media on request
  * completion.
  *
  * If the device doesn't have writeback cache, FLUSH and FUA don't make any
  * difference.  The requests are either completed immediately if there's no
  * data or executed as normal requests otherwise.
  *
  * If the device has writeback cache and supports FUA, REQ_FLUSH is
  * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
  *
  * If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
  * translated to PREFLUSH and REQ_FUA to POSTFLUSH.
  *
  * The actual execution of flush is double buffered.  Whenever a request
  * needs to execute PRE or POSTFLUSH, it queues at
  * q->flush_queue[q->flush_pending_idx].  Once certain criteria are met, a
  * flush is issued and the pending_idx is toggled.  When the flush
  * completes, all the requests which were pending are proceeded to the next
  * step.  This allows arbitrary merging of different types of FLUSH/FUA
  * requests.
  *
  * Currently, the following conditions are used to determine when to issue
  * flush.
  *
  * C1. At any given time, only one flush shall be in progress.  This makes
  *     double buffering sufficient.
  *
  * C2. Flush is deferred if any request is executing DATA of its sequence.
  *     This avoids issuing separate POSTFLUSHes for requests which shared
  *     PREFLUSH.
  *
  * C3. The second condition is ignored if there is a request which has
  *     waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
  *     starvation in the unlikely case where there are continuous stream of
  *     FUA (without FLUSH) requests.
  *
  * For devices which support FUA, it isn't clear whether C2 (and thus C3)
  * is beneficial.
  *
  * Note that a sequenced FLUSH/FUA request with DATA is completed twice.
  * Once while executing DATA and again after the whole sequence is
  * complete.  The first completion updates the contained bio but doesn't
  * finish it so that the bio submitter is notified only after the whole
  * sequence is complete.  This is implemented by testing REQ_FLUSH_SEQ in
  * req_bio_endio().
  *
  * The above peculiarity requires that each FLUSH/FUA request has only one
  * bio attached to it, which is guaranteed as they aren't allowed to be
  * merged in the usual way.
  */

 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/bio.h>
 #include <linux/blkdev.h>
 #include <linux/gfp.h>

 #include "blk.h"

 /* FLUSH/FUA sequences */
 enum {
 	REQ_FSEQ_PREFLUSH	= (1 << 0), /* pre-flushing in progress */
 	REQ_FSEQ_DATA		= (1 << 1), /* data write in progress */
 	REQ_FSEQ_POSTFLUSH	= (1 << 2), /* post-flushing in progress */
 	REQ_FSEQ_DONE		= (1 << 3),

 	REQ_FSEQ_ACTIONS	= REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
 				  REQ_FSEQ_POSTFLUSH,

 	/*
 	 * If flush has been pending longer than the following timeout,
 	 * it's issued even if flush_data requests are still in flight.
 	 */
 	FLUSH_PENDING_TIMEOUT	= 5 * HZ,
 };

 static bool blk_kick_flush(struct request_queue *q);

 static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
 {
 	unsigned int policy = 0;

 	if (blk_rq_sectors(rq))
 		policy |= REQ_FSEQ_DATA;

 	if (fflags & REQ_FLUSH) {
 		if (rq->cmd_flags & REQ_FLUSH)
 			policy |= REQ_FSEQ_PREFLUSH;
 		if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
 			policy |= REQ_FSEQ_POSTFLUSH;
 	}
 	return policy;
 }

 static unsigned int blk_flush_cur_seq(struct request *rq)
 {
 	return 1 << ffz(rq->flush.seq);
 }

 static void blk_flush_restore_request(struct request *rq)
 {
 	/*
 	 * After flush data completion, @rq->bio is %NULL but we need to
 	 * complete the bio again.  @rq->biotail is guaranteed to equal the
 	 * original @rq->bio.  Restore it.
 	 */
 	rq->bio = rq->biotail;

 	/* make @rq a normal request */
 	rq->cmd_flags &= ~REQ_FLUSH_SEQ;
 	rq->end_io = rq->flush.saved_end_io;
 }

 /**
  * blk_flush_complete_seq - complete flush sequence
  * @rq: FLUSH/FUA request being sequenced
  * @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
  * @error: whether an error occurred
  *
  * @rq just completed @seq part of its flush sequence, record the
  * completion and trigger the next step.
  *
  * CONTEXT:
  * spin_lock_irq(q->queue_lock)
  *
  * RETURNS:
  * %true if requests were added to the dispatch queue, %false otherwise.
  */
 static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
 				   int error)
 {
 	struct request_queue *q = rq->q;
 	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
 	bool queued = false;

 	BUG_ON(rq->flush.seq & seq);
 	rq->flush.seq |= seq;

 	if (likely(!error))
 		seq = blk_flush_cur_seq(rq);
 	else
 		seq = REQ_FSEQ_DONE;

 	switch (seq) {
 	case REQ_FSEQ_PREFLUSH:
 	case REQ_FSEQ_POSTFLUSH:
 		/* queue for flush */
 		if (list_empty(pending))
 			q->flush_pending_since = jiffies;
 		list_move_tail(&rq->flush.list, pending);
 		break;

 	case REQ_FSEQ_DATA:
 		list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
 		list_add(&rq->queuelist, &q->queue_head);
 		queued = true;
 		break;

 	case REQ_FSEQ_DONE:
 		/*
 		 * @rq was previously adjusted by blk_flush_issue() for
 		 * flush sequencing and may already have gone through the
 		 * flush data request completion path.  Restore @rq for
 		 * normal completion and end it.
 		 */
 		BUG_ON(!list_empty(&rq->queuelist));
 		list_del_init(&rq->flush.list);
 		blk_flush_restore_request(rq);
 		__blk_end_request_all(rq, error);
 		break;

 	default:
 		BUG();
 	}

 	return blk_kick_flush(q) | queued;
 }

 static void flush_end_io(struct request *flush_rq, int error)
 {
 	struct request_queue *q = flush_rq->q;
 	struct list_head *running = &q->flush_queue[q->flush_running_idx];
 	bool queued = false;
 	struct request *rq, *n;

 	BUG_ON(q->flush_pending_idx == q->flush_running_idx);

 	/* account completion of the flush request */
 	q->flush_running_idx ^= 1;
 	elv_completed_request(q, flush_rq);

 	/* and push the waiting requests to the next stage */
 	list_for_each_entry_safe(rq, n, running, flush.list) {
 		unsigned int seq = blk_flush_cur_seq(rq);

 		BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
 		queued |= blk_flush_complete_seq(rq, seq, error);
 	}

 	/*
 	 * Kick the queue to avoid stall for two cases:
 	 * 1. Moving a request silently to empty queue_head may stall the
 	 * queue.
 	 * 2. When flush request is running in non-queueable queue, the
 	 * queue is hold. Restart the queue after flush request is finished
 	 * to avoid stall.
 	 * This function is called from request completion path and calling
 	 * directly into request_fn may confuse the driver.  Always use
 	 * kblockd.
 	 */
 	if (queued || q->flush_queue_delayed)
 		blk_run_queue_async(q);
 	q->flush_queue_delayed = 0;
 }

 /**
  * blk_kick_flush - consider issuing flush request
  * @q: request_queue being kicked
  *
  * Flush related states of @q have changed, consider issuing flush request.
  * Please read the comment at the top of this file for more info.
  *
  * CONTEXT:
  * spin_lock_irq(q->queue_lock)
  *
  * RETURNS:
  * %true if flush was issued, %false otherwise.
  */
 static bool blk_kick_flush(struct request_queue *q)
 {
 	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
 	struct request *first_rq =
 		list_first_entry(pending, struct request, flush.list);

 	/* C1 described at the top of this file */
 	if (q->flush_pending_idx != q->flush_running_idx || list_empty(pending))
 		return false;

 	/* C2 and C3 */
 	if (!list_empty(&q->flush_data_in_flight) &&
 	    time_before(jiffies,
 			q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
 		return false;

 	/*
 	 * Issue flush and toggle pending_idx.  This makes pending_idx
 	 * different from running_idx, which means flush is in flight.
 	 */
 	blk_rq_init(q, &q->flush_rq);
 	q->flush_rq.cmd_type = REQ_TYPE_FS;
 	q->flush_rq.cmd_flags = WRITE_FLUSH | REQ_FLUSH_SEQ;
 	q->flush_rq.rq_disk = first_rq->rq_disk;
 	q->flush_rq.end_io = flush_end_io;

 	q->flush_pending_idx ^= 1;
 	list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
 	return true;
 }

 static void flush_data_end_io(struct request *rq, int error)
 {
 	struct request_queue *q = rq->q;

 	/*
 	 * After populating an empty queue, kick it to avoid stall.  Read
 	 * the comment in flush_end_io().
 	 */
 	if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))
 		blk_run_queue_async(q);
 }

 /**
  * blk_insert_flush - insert a new FLUSH/FUA request
  * @rq: request to insert
  *
  * To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
  * @rq is being submitted.  Analyze what needs to be done and put it on the
  * right queue.
  *
  * CONTEXT:
  * spin_lock_irq(q->queue_lock)
  */
 void blk_insert_flush(struct request *rq)
 {
 	struct request_queue *q = rq->q;
 	unsigned int fflags = q->flush_flags;	/* may change, cache */
 	unsigned int policy = blk_flush_policy(fflags, rq);

 	/*
 	 * @policy now records what operations need to be done.  Adjust
 	 * REQ_FLUSH and FUA for the driver.
 	 */
 	rq->cmd_flags &= ~REQ_FLUSH;
 	if (!(fflags & REQ_FUA))
 		rq->cmd_flags &= ~REQ_FUA;

 	/*
 	 * An empty flush handed down from a stacking driver may
 	 * translate into nothing if the underlying device does not
 	 * advertise a write-back cache.  In this case, simply
 	 * complete the request.
 	 */
 	if (!policy) {
 		__blk_end_bidi_request(rq, 0, 0, 0);
 		return;
 	}

 	BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */

 	/*
 	 * If there's data but flush is not necessary, the request can be
 	 * processed directly without going through flush machinery.  Queue
 	 * for normal execution.
 	 */
 	if ((policy & REQ_FSEQ_DATA) &&
 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
 		list_add_tail(&rq->queuelist, &q->queue_head);
 		return;
 	}

 	/*
 	 * @rq should go through flush machinery.  Mark it part of flush
 	 * sequence and submit for further processing.
 	 */
 	memset(&rq->flush, 0, sizeof(rq->flush));
 	INIT_LIST_HEAD(&rq->flush.list);
 	rq->cmd_flags |= REQ_FLUSH_SEQ;
 	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
 	rq->end_io = flush_data_end_io;

 	blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
 }

 /**
  * blk_abort_flushes - @q is being aborted, abort flush requests
  * @q: request_queue being aborted
  *
  * To be called from elv_abort_queue().  @q is being aborted.  Prepare all
  * FLUSH/FUA requests for abortion.
  *
  * CONTEXT:
  * spin_lock_irq(q->queue_lock)
  */
 void blk_abort_flushes(struct request_queue *q)
 {
 	struct request *rq, *n;
 	int i;

 	/*
 	 * Requests in flight for data are already owned by the dispatch
 	 * queue or the device driver.  Just restore for normal completion.
 	 */
 	list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
 		list_del_init(&rq->flush.list);
 		blk_flush_restore_request(rq);
 	}

 	/*
 	 * We need to give away requests on flush queues.  Restore for
 	 * normal completion and put them on the dispatch queue.
 	 */
 	for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
 		list_for_each_entry_safe(rq, n, &q->flush_queue[i],
 					 flush.list) {
 			list_del_init(&rq->flush.list);
 			blk_flush_restore_request(rq);
 			list_add_tail(&rq->queuelist, &q->queue_head);
 		}
 	}
 }

 static void bio_end_flush(struct bio *bio, int err)
 {
 	if (err)
 		clear_bit(BIO_UPTODATE, &bio->bi_flags);
 	if (bio->bi_private)
 		complete(bio->bi_private);
 	bio_put(bio);
 }

 /**
  * blkdev_issue_flush - queue a flush
  * @bdev:	blockdev to issue flush for
  * @gfp_mask:	memory allocation flags (for bio_alloc)
  * @error_sector:	error sector
  *
  * Description:
  *    Issue a flush for the block device in question. Caller can supply
  *    room for storing the error offset in case of a flush error, if they
  *    wish to. If WAIT flag is not passed then caller may check only what
  *    request was pushed in some internal queue for later handling.
  */
 int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 		sector_t *error_sector)
 {
 	DECLARE_COMPLETION_ONSTACK(wait);
 	struct request_queue *q;
 	struct bio *bio;
 	int ret = 0;

 	if (bdev->bd_disk == NULL)
 		return -ENXIO;

 	q = bdev_get_queue(bdev);
 	if (!q)
 		return -ENXIO;

 	/*
 	 * some block devices may not have their queue correctly set up here
 	 * (e.g. loop device without a backing file) and so issuing a flush
 	 * here will panic. Ensure there is a request function before issuing
 	 * the flush.
 	 */
 	if (!q->make_request_fn)
 		return -ENXIO;

 	bio = bio_alloc(gfp_mask, 0);
 	bio->bi_end_io = bio_end_flush;
 	bio->bi_bdev = bdev;
 	bio->bi_private = &wait;

 	bio_get(bio);
 	submit_bio(WRITE_FLUSH, bio);
 	wait_for_completion(&wait);

 	/*
 	 * The driver must store the error location in ->bi_sector, if
 	 * it supports it. For non-stacked drivers, this should be
 	 * copied from blk_rq_pos(rq).
 	 */
 	if (error_sector)
                *error_sector = bio->bi_sector;

 	if (!bio_flagged(bio, BIO_UPTODATE))
 		ret = -EIO;

 	bio_put(bio);
 	return ret;
 }
 EXPORT_SYMBOL(blkdev_issue_flush);
	/*
	* Functions to sequence FLUSH and FUA writes.
	*
	* Copyright (C) 2011 Max Planck Institute for Gravitational Physics
	* Copyright (C) 2011 Tejun Heo <tj@kernel.org>
	*
	* This file is released under the GPLv2.
	*
	* REQ_{FLUSH\|FUA} requests are decomposed to sequences consisted of three
	* optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
	* properties and hardware capability.
	*
	* If a request doesn't have data, only REQ_FLUSH makes sense, which
	* indicates a simple flush request. If there is data, REQ_FLUSH indicates
	* that the device cache should be flushed before the data is executed, and
	* REQ_FUA means that the data must be on non-volatile media on request
	* completion.
	*
	* If the device doesn't have writeback cache, FLUSH and FUA don't make any
	* difference. The requests are either completed immediately if there's no
	* data or executed as normal requests otherwise.
	*
	* If the device has writeback cache and supports FUA, REQ_FLUSH is
	* translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
	*
	* If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
	* translated to PREFLUSH and REQ_FUA to POSTFLUSH.
	*
	* The actual execution of flush is double buffered. Whenever a request
	* needs to execute PRE or POSTFLUSH, it queues at
	* q->flush_queue[q->flush_pending_idx]. Once certain criteria are met, a
	* flush is issued and the pending_idx is toggled. When the flush
	* completes, all the requests which were pending are proceeded to the next
	* step. This allows arbitrary merging of different types of FLUSH/FUA
	* requests.
	*
	* Currently, the following conditions are used to determine when to issue
	* flush.
	*
	* C1. At any given time, only one flush shall be in progress. This makes
	* double buffering sufficient.
	*
	* C2. Flush is deferred if any request is executing DATA of its sequence.
	* This avoids issuing separate POSTFLUSHes for requests which shared
	* PREFLUSH.
	*
	* C3. The second condition is ignored if there is a request which has
	* waited longer than FLUSH_PENDING_TIMEOUT. This is to avoid
	* starvation in the unlikely case where there are continuous stream of
	* FUA (without FLUSH) requests.
	*
	* For devices which support FUA, it isn't clear whether C2 (and thus C3)
	* is beneficial.
	*
	* Note that a sequenced FLUSH/FUA request with DATA is completed twice.
	* Once while executing DATA and again after the whole sequence is
	* complete. The first completion updates the contained bio but doesn't
	* finish it so that the bio submitter is notified only after the whole
	* sequence is complete. This is implemented by testing REQ_FLUSH_SEQ in
	* req_bio_endio().
	*
	* The above peculiarity requires that each FLUSH/FUA request has only one
	* bio attached to it, which is guaranteed as they aren't allowed to be
	* merged in the usual way.
	*/

	#include <linux/kernel.h>
	#include <linux/module.h>
	#include <linux/bio.h>
	#include <linux/blkdev.h>
	#include <linux/gfp.h>

	#include "blk.h"

	/* FLUSH/FUA sequences */
	enum {
	REQ_FSEQ_PREFLUSH = (1 << 0), /* pre-flushing in progress */
	REQ_FSEQ_DATA = (1 << 1), /* data write in progress */
	REQ_FSEQ_POSTFLUSH = (1 << 2), /* post-flushing in progress */
	REQ_FSEQ_DONE = (1 << 3),

	REQ_FSEQ_ACTIONS = REQ_FSEQ_PREFLUSH \| REQ_FSEQ_DATA \|
	REQ_FSEQ_POSTFLUSH,

	/*
	* If flush has been pending longer than the following timeout,
	* it's issued even if flush_data requests are still in flight.
	*/
	FLUSH_PENDING_TIMEOUT = 5 * HZ,
	};

	static bool blk_kick_flush(struct request_queue *q);

	static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
	{
	unsigned int policy = 0;

	if (blk_rq_sectors(rq))
	policy \|= REQ_FSEQ_DATA;

	if (fflags & REQ_FLUSH) {
	if (rq->cmd_flags & REQ_FLUSH)
	policy \|= REQ_FSEQ_PREFLUSH;
	if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
	policy \|= REQ_FSEQ_POSTFLUSH;
	}
	return policy;
	}

	static unsigned int blk_flush_cur_seq(struct request *rq)
	{
	return 1 << ffz(rq->flush.seq);
	}

	static void blk_flush_restore_request(struct request *rq)
	{
	/*
	* After flush data completion, @rq->bio is %NULL but we need to
	* complete the bio again. @rq->biotail is guaranteed to equal the
	* original @rq->bio. Restore it.
	*/
	rq->bio = rq->biotail;

	/* make @rq a normal request */
	rq->cmd_flags &= ~REQ_FLUSH_SEQ;
	rq->end_io = rq->flush.saved_end_io;
	}

	/**
	* blk_flush_complete_seq - complete flush sequence
	* @rq: FLUSH/FUA request being sequenced
	* @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
	* @error: whether an error occurred
	*
	* @rq just completed @seq part of its flush sequence, record the
	* completion and trigger the next step.
	*
	* CONTEXT:
	* spin_lock_irq(q->queue_lock)
	*
	* RETURNS:
	* %true if requests were added to the dispatch queue, %false otherwise.
	*/
	static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
	int error)
	{
	struct request_queue *q = rq->q;
	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
	bool queued = false;

	BUG_ON(rq->flush.seq & seq);
	rq->flush.seq \|= seq;

	if (likely(!error))
	seq = blk_flush_cur_seq(rq);
	else
	seq = REQ_FSEQ_DONE;

	switch (seq) {
	case REQ_FSEQ_PREFLUSH:
	case REQ_FSEQ_POSTFLUSH:
	/* queue for flush */
	if (list_empty(pending))
	q->flush_pending_since = jiffies;
	list_move_tail(&rq->flush.list, pending);
	break;

	case REQ_FSEQ_DATA:
	list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
	list_add(&rq->queuelist, &q->queue_head);
	queued = true;
	break;

	case REQ_FSEQ_DONE:
	/*
	* @rq was previously adjusted by blk_flush_issue() for
	* flush sequencing and may already have gone through the
	* flush data request completion path. Restore @rq for
	* normal completion and end it.
	*/
	BUG_ON(!list_empty(&rq->queuelist));
	list_del_init(&rq->flush.list);
	blk_flush_restore_request(rq);
	__blk_end_request_all(rq, error);
	break;

	default:
	BUG();
	}

	return blk_kick_flush(q) \| queued;
	}

	static void flush_end_io(struct request *flush_rq, int error)
	{
	struct request_queue *q = flush_rq->q;
	struct list_head *running = &q->flush_queue[q->flush_running_idx];
	bool queued = false;
	struct request rq, n;

	BUG_ON(q->flush_pending_idx == q->flush_running_idx);

	/* account completion of the flush request */
	q->flush_running_idx ^= 1;
	elv_completed_request(q, flush_rq);

	/* and push the waiting requests to the next stage */
	list_for_each_entry_safe(rq, n, running, flush.list) {
	unsigned int seq = blk_flush_cur_seq(rq);

	BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
	queued \|= blk_flush_complete_seq(rq, seq, error);
	}

	/*
	* Kick the queue to avoid stall for two cases:
	* 1. Moving a request silently to empty queue_head may stall the
	* queue.
	* 2. When flush request is running in non-queueable queue, the
	* queue is hold. Restart the queue after flush request is finished
	* to avoid stall.
	* This function is called from request completion path and calling
	* directly into request_fn may confuse the driver. Always use
	* kblockd.
	*/
	if (queued \|\| q->flush_queue_delayed)
	blk_run_queue_async(q);
	q->flush_queue_delayed = 0;
	}

	/**
	* blk_kick_flush - consider issuing flush request
	* @q: request_queue being kicked
	*
	* Flush related states of @q have changed, consider issuing flush request.
	* Please read the comment at the top of this file for more info.
	*
	* CONTEXT:
	* spin_lock_irq(q->queue_lock)
	*
	* RETURNS:
	* %true if flush was issued, %false otherwise.
	*/
	static bool blk_kick_flush(struct request_queue *q)
	{
	struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
	struct request *first_rq =
	list_first_entry(pending, struct request, flush.list);

	/* C1 described at the top of this file */
	if (q->flush_pending_idx != q->flush_running_idx \|\| list_empty(pending))
	return false;

	/* C2 and C3 */
	if (!list_empty(&q->flush_data_in_flight) &&
	time_before(jiffies,
	q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
	return false;

	/*
	* Issue flush and toggle pending_idx. This makes pending_idx
	* different from running_idx, which means flush is in flight.
	*/
	blk_rq_init(q, &q->flush_rq);
	q->flush_rq.cmd_type = REQ_TYPE_FS;
	q->flush_rq.cmd_flags = WRITE_FLUSH \| REQ_FLUSH_SEQ;
	q->flush_rq.rq_disk = first_rq->rq_disk;
	q->flush_rq.end_io = flush_end_io;

	q->flush_pending_idx ^= 1;
	list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
	return true;
	}

	static void flush_data_end_io(struct request *rq, int error)
	{
	struct request_queue *q = rq->q;

	/*
	* After populating an empty queue, kick it to avoid stall. Read
	* the comment in flush_end_io().
	*/
	if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))
	blk_run_queue_async(q);
	}

	/**
	* blk_insert_flush - insert a new FLUSH/FUA request
	* @rq: request to insert
	*
	* To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
	* @rq is being submitted. Analyze what needs to be done and put it on the
	* right queue.
	*
	* CONTEXT:
	* spin_lock_irq(q->queue_lock)
	*/
	void blk_insert_flush(struct request *rq)
	{
	struct request_queue *q = rq->q;
	unsigned int fflags = q->flush_flags; /* may change, cache */
	unsigned int policy = blk_flush_policy(fflags, rq);

	/*
	* @policy now records what operations need to be done. Adjust
	* REQ_FLUSH and FUA for the driver.
	*/
	rq->cmd_flags &= ~REQ_FLUSH;
	if (!(fflags & REQ_FUA))
	rq->cmd_flags &= ~REQ_FUA;

	/*
	* An empty flush handed down from a stacking driver may
	* translate into nothing if the underlying device does not
	* advertise a write-back cache. In this case, simply
	* complete the request.
	*/
	if (!policy) {
	__blk_end_bidi_request(rq, 0, 0, 0);
	return;
	}

	BUG_ON(rq->bio != rq->biotail); /assumes zero or single bio rq /

	/*
	* If there's data but flush is not necessary, the request can be
	* processed directly without going through flush machinery. Queue
	* for normal execution.
	*/
	if ((policy & REQ_FSEQ_DATA) &&
	!(policy & (REQ_FSEQ_PREFLUSH \| REQ_FSEQ_POSTFLUSH))) {
	list_add_tail(&rq->queuelist, &q->queue_head);
	return;
	}

	/*
	* @rq should go through flush machinery. Mark it part of flush
	* sequence and submit for further processing.
	*/
	memset(&rq->flush, 0, sizeof(rq->flush));
	INIT_LIST_HEAD(&rq->flush.list);
	rq->cmd_flags \|= REQ_FLUSH_SEQ;
	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
	rq->end_io = flush_data_end_io;

	blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
	}

	/**
	* blk_abort_flushes - @q is being aborted, abort flush requests
	* @q: request_queue being aborted
	*
	* To be called from elv_abort_queue(). @q is being aborted. Prepare all
	* FLUSH/FUA requests for abortion.
	*
	* CONTEXT:
	* spin_lock_irq(q->queue_lock)
	*/
	void blk_abort_flushes(struct request_queue *q)
	{
	struct request rq, n;
	int i;

	/*
	* Requests in flight for data are already owned by the dispatch
	* queue or the device driver. Just restore for normal completion.
	*/
	list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
	list_del_init(&rq->flush.list);
	blk_flush_restore_request(rq);
	}

	/*
	* We need to give away requests on flush queues. Restore for
	* normal completion and put them on the dispatch queue.
	*/
	for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
	list_for_each_entry_safe(rq, n, &q->flush_queue[i],
	flush.list) {
	list_del_init(&rq->flush.list);
	blk_flush_restore_request(rq);
	list_add_tail(&rq->queuelist, &q->queue_head);
	}
	}
	}

	static void bio_end_flush(struct bio *bio, int err)
	{
	if (err)
	clear_bit(BIO_UPTODATE, &bio->bi_flags);
	if (bio->bi_private)
	complete(bio->bi_private);
	bio_put(bio);
	}

	/**
	* blkdev_issue_flush - queue a flush
	* @bdev: blockdev to issue flush for
	* @gfp_mask: memory allocation flags (for bio_alloc)
	* @error_sector: error sector
	*
	* Description:
	* Issue a flush for the block device in question. Caller can supply
	* room for storing the error offset in case of a flush error, if they
	* wish to. If WAIT flag is not passed then caller may check only what
	* request was pushed in some internal queue for later handling.
	*/
	int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
	sector_t *error_sector)
	{
	DECLARE_COMPLETION_ONSTACK(wait);
	struct request_queue *q;
	struct bio *bio;
	int ret = 0;

	if (bdev->bd_disk == NULL)
	return -ENXIO;

	q = bdev_get_queue(bdev);
	if (!q)
	return -ENXIO;

	/*
	* some block devices may not have their queue correctly set up here
	* (e.g. loop device without a backing file) and so issuing a flush
	* here will panic. Ensure there is a request function before issuing
	* the flush.
	*/
	if (!q->make_request_fn)
	return -ENXIO;

	bio = bio_alloc(gfp_mask, 0);
	bio->bi_end_io = bio_end_flush;
	bio->bi_bdev = bdev;
	bio->bi_private = &wait;

	bio_get(bio);
	submit_bio(WRITE_FLUSH, bio);
	wait_for_completion(&wait);

	/*
	* The driver must store the error location in ->bi_sector, if
	* it supports it. For non-stacked drivers, this should be
	* copied from blk_rq_pos(rq).
	*/
	if (error_sector)
	*error_sector = bio->bi_sector;

	if (!bio_flagged(bio, BIO_UPTODATE))
	ret = -EIO;

	bio_put(bio);
	return ret;
	}
	EXPORT_SYMBOL(blkdev_issue_flush);