/*
 * 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 (fflags & REQ_FLUSH) {
                if (rq->cmd_flags & REQ_FLUSH)
                        policy |= REQ_FSEQ_PREFLUSH;
                if (blk_rq_sectors(rq))
                        policy |= REQ_FSEQ_DATA;
                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 = NULL;
}

/**
 * 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);
        }

        /*
         * Moving a request silently to empty queue_head may stall the
         * queue.  Kick the queue in those cases.  This function is called
         * from request completion path and calling directly into
         * request_fn may confuse the driver.  Always use kblockd.
         */
        if (queued)
                blk_run_queue_async(q);
}

/**
 * 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);

        BUG_ON(rq->end_io);
        BUG_ON(!rq->bio || rq->bio != rq->biotail);

        /*
         * @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;

        /*
         * 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->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);