[mv-sheeva.git] / drivers / block / drbd / drbd_req.c

/*
   drbd_req.c

   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>.
   Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

 */

#include <linux/module.h>

#include <linux/slab.h>
#include <linux/drbd.h>
#include "drbd_int.h"
#include "drbd_req.h"


/* Update disk stats at start of I/O request */
static void _drbd_start_io_acct(struct drbd_conf *mdev, struct drbd_request *req, struct bio *bio)
{
        const int rw = bio_data_dir(bio);
        int cpu;
        cpu = part_stat_lock();
        part_stat_inc(cpu, &mdev->vdisk->part0, ios[rw]);
        part_stat_add(cpu, &mdev->vdisk->part0, sectors[rw], bio_sectors(bio));
        part_inc_in_flight(&mdev->vdisk->part0, rw);
        part_stat_unlock();
}

/* Update disk stats when completing request upwards */
static void _drbd_end_io_acct(struct drbd_conf *mdev, struct drbd_request *req)
{
        int rw = bio_data_dir(req->master_bio);
        unsigned long duration = jiffies - req->start_time;
        int cpu;
        cpu = part_stat_lock();
        part_stat_add(cpu, &mdev->vdisk->part0, ticks[rw], duration);
        part_round_stats(cpu, &mdev->vdisk->part0);
        part_dec_in_flight(&mdev->vdisk->part0, rw);
        part_stat_unlock();
}

static void _req_is_done(struct drbd_conf *mdev, struct drbd_request *req, const int rw)
{
        const unsigned long s = req->rq_state;

        /* remove it from the transfer log.
         * well, only if it had been there in the first
         * place... if it had not (local only or conflicting
         * and never sent), it should still be "empty" as
         * initialized in drbd_req_new(), so we can list_del() it
         * here unconditionally */
        list_del(&req->tl_requests);

        /* if it was a write, we may have to set the corresponding
         * bit(s) out-of-sync first. If it had a local part, we need to
         * release the reference to the activity log. */
        if (rw == WRITE) {
                /* Set out-of-sync unless both OK flags are set
                 * (local only or remote failed).
                 * Other places where we set out-of-sync:
                 * READ with local io-error */
                if (!(s & RQ_NET_OK) || !(s & RQ_LOCAL_OK))
                        drbd_set_out_of_sync(mdev, req->sector, req->size);

                if ((s & RQ_NET_OK) && (s & RQ_LOCAL_OK) && (s & RQ_NET_SIS))
                        drbd_set_in_sync(mdev, req->sector, req->size);

                /* one might be tempted to move the drbd_al_complete_io
                 * to the local io completion callback drbd_endio_pri.
                 * but, if this was a mirror write, we may only
                 * drbd_al_complete_io after this is RQ_NET_DONE,
                 * otherwise the extent could be dropped from the al
                 * before it has actually been written on the peer.
                 * if we crash before our peer knows about the request,
                 * but after the extent has been dropped from the al,
                 * we would forget to resync the corresponding extent.
                 */
                if (s & RQ_LOCAL_MASK) {
                        if (get_ldev_if_state(mdev, D_FAILED)) {
                                drbd_al_complete_io(mdev, req->sector);
                                put_ldev(mdev);
                        } else if (__ratelimit(&drbd_ratelimit_state)) {
                                dev_warn(DEV, "Should have called drbd_al_complete_io(, %llu), "
                                     "but my Disk seems to have failed :(\n",
                                     (unsigned long long) req->sector);
                        }
                }
        }

        drbd_req_free(req);
}

static void queue_barrier(struct drbd_conf *mdev)
{
        struct drbd_tl_epoch *b;

        /* We are within the req_lock. Once we queued the barrier for sending,
         * we set the CREATE_BARRIER bit. It is cleared as soon as a new
         * barrier/epoch object is added. This is the only place this bit is
         * set. It indicates that the barrier for this epoch is already queued,
         * and no new epoch has been created yet. */
        if (test_bit(CREATE_BARRIER, &mdev->flags))
                return;

        b = mdev->newest_tle;
        b->w.cb = w_send_barrier;
        /* inc_ap_pending done here, so we won't
         * get imbalanced on connection loss.
         * dec_ap_pending will be done in got_BarrierAck
         * or (on connection loss) in tl_clear.  */
        inc_ap_pending(mdev);
        drbd_queue_work(&mdev->data.work, &b->w);
        set_bit(CREATE_BARRIER, &mdev->flags);
}

static void _about_to_complete_local_write(struct drbd_conf *mdev,
        struct drbd_request *req)
{
        const unsigned long s = req->rq_state;
        struct drbd_request *i;
        struct drbd_epoch_entry *e;
        struct hlist_node *n;
        struct hlist_head *slot;

        /* before we can signal completion to the upper layers,
         * we may need to close the current epoch */
        if (mdev->state.conn >= C_CONNECTED &&
            req->epoch == mdev->newest_tle->br_number)
                queue_barrier(mdev);

        /* we need to do the conflict detection stuff,
         * if we have the ee_hash (two_primaries) and
         * this has been on the network */
        if ((s & RQ_NET_DONE) && mdev->ee_hash != NULL) {
                const sector_t sector = req->sector;
                const int size = req->size;

                /* ASSERT:
                 * there must be no conflicting requests, since
                 * they must have been failed on the spot */
#define OVERLAPS overlaps(sector, size, i->sector, i->size)
                slot = tl_hash_slot(mdev, sector);
                hlist_for_each_entry(i, n, slot, colision) {
                        if (OVERLAPS) {
                                dev_alert(DEV, "LOGIC BUG: completed: %p %llus +%u; "
                                      "other: %p %llus +%u\n",
                                      req, (unsigned long long)sector, size,
                                      i, (unsigned long long)i->sector, i->size);
                        }
                }

                /* maybe "wake" those conflicting epoch entries
                 * that wait for this request to finish.
                 *
                 * currently, there can be only _one_ such ee
                 * (well, or some more, which would be pending
                 * P_DISCARD_ACK not yet sent by the asender...),
                 * since we block the receiver thread upon the
                 * first conflict detection, which will wait on
                 * misc_wait.  maybe we want to assert that?
                 *
                 * anyways, if we found one,
                 * we just have to do a wake_up.  */
#undef OVERLAPS
#define OVERLAPS overlaps(sector, size, e->sector, e->size)
                slot = ee_hash_slot(mdev, req->sector);
                hlist_for_each_entry(e, n, slot, colision) {
                        if (OVERLAPS) {
                                wake_up(&mdev->misc_wait);
                                break;
                        }
                }
        }
#undef OVERLAPS
}

void complete_master_bio(struct drbd_conf *mdev,
                struct bio_and_error *m)
{
        bio_endio(m->bio, m->error);
        dec_ap_bio(mdev);
}

/* Helper for __req_mod().
 * Set m->bio to the master bio, if it is fit to be completed,
 * or leave it alone (it is initialized to NULL in __req_mod),
 * if it has already been completed, or cannot be completed yet.
 * If m->bio is set, the error status to be returned is placed in m->error.
 */
void _req_may_be_done(struct drbd_request *req, struct bio_and_error *m)
{
        const unsigned long s = req->rq_state;
        struct drbd_conf *mdev = req->mdev;
        /* only WRITES may end up here without a master bio (on barrier ack) */
        int rw = req->master_bio ? bio_data_dir(req->master_bio) : WRITE;

        /* we must not complete the master bio, while it is
         *      still being processed by _drbd_send_zc_bio (drbd_send_dblock)
         *      not yet acknowledged by the peer
         *      not yet completed by the local io subsystem
         * these flags may get cleared in any order by
         *      the worker,
         *      the receiver,
         *      the bio_endio completion callbacks.
         */
        if (s & RQ_NET_QUEUED)
                return;
        if (s & RQ_NET_PENDING)
                return;
        if (s & RQ_LOCAL_PENDING)
                return;

        if (req->master_bio) {
                /* this is data_received (remote read)
                 * or protocol C P_WRITE_ACK
                 * or protocol B P_RECV_ACK
                 * or protocol A "handed_over_to_network" (SendAck)
                 * or canceled or failed,
                 * or killed from the transfer log due to connection loss.
                 */

                /*
                 * figure out whether to report success or failure.
                 *
                 * report success when at least one of the operations succeeded.
                 * or, to put the other way,
                 * only report failure, when both operations failed.
                 *
                 * what to do about the failures is handled elsewhere.
                 * what we need to do here is just: complete the master_bio.
                 *
                 * local completion error, if any, has been stored as ERR_PTR
                 * in private_bio within drbd_endio_pri.
                 */
                int ok = (s & RQ_LOCAL_OK) || (s & RQ_NET_OK);
                int error = PTR_ERR(req->private_bio);

                /* remove the request from the conflict detection
                 * respective block_id verification hash */
                if (!hlist_unhashed(&req->colision))
                        hlist_del(&req->colision);
                else
                        D_ASSERT((s & RQ_NET_MASK) == 0);

                /* for writes we need to do some extra housekeeping */
                if (rw == WRITE)
                        _about_to_complete_local_write(mdev, req);

                /* Update disk stats */
                _drbd_end_io_acct(mdev, req);

                m->error = ok ? 0 : (error ?: -EIO);
                m->bio = req->master_bio;
                req->master_bio = NULL;
        }

        if ((s & RQ_NET_MASK) == 0 || (s & RQ_NET_DONE)) {
                /* this is disconnected (local only) operation,
                 * or protocol C P_WRITE_ACK,
                 * or protocol A or B P_BARRIER_ACK,
                 * or killed from the transfer log due to connection loss. */
                _req_is_done(mdev, req, rw);
        }
        /* else: network part and not DONE yet. that is
         * protocol A or B, barrier ack still pending... */
}

static void _req_may_be_done_not_susp(struct drbd_request *req, struct bio_and_error *m)
{
        struct drbd_conf *mdev = req->mdev;

        if (!mdev->state.susp)
                _req_may_be_done(req, m);
}

/*
 * checks whether there was an overlapping request
 * or ee already registered.
 *
 * if so, return 1, in which case this request is completed on the spot,
 * without ever being submitted or send.
 *
 * return 0 if it is ok to submit this request.
 *
 * NOTE:
 * paranoia: assume something above us is broken, and issues different write
 * requests for the same block simultaneously...
 *
 * To ensure these won't be reordered differently on both nodes, resulting in
 * diverging data sets, we discard the later one(s). Not that this is supposed
 * to happen, but this is the rationale why we also have to check for
 * conflicting requests with local origin, and why we have to do so regardless
 * of whether we allowed multiple primaries.
 *
 * BTW, in case we only have one primary, the ee_hash is empty anyways, and the
 * second hlist_for_each_entry becomes a noop. This is even simpler than to
 * grab a reference on the net_conf, and check for the two_primaries flag...
 */
static int _req_conflicts(struct drbd_request *req)
{
        struct drbd_conf *mdev = req->mdev;
        const sector_t sector = req->sector;
        const int size = req->size;
        struct drbd_request *i;
        struct drbd_epoch_entry *e;
        struct hlist_node *n;
        struct hlist_head *slot;

        D_ASSERT(hlist_unhashed(&req->colision));

        if (!get_net_conf(mdev))
                return 0;

        /* BUG_ON */
        ERR_IF (mdev->tl_hash_s == 0)
                goto out_no_conflict;
        BUG_ON(mdev->tl_hash == NULL);

#define OVERLAPS overlaps(i->sector, i->size, sector, size)
        slot = tl_hash_slot(mdev, sector);
        hlist_for_each_entry(i, n, slot, colision) {
                if (OVERLAPS) {
                        dev_alert(DEV, "%s[%u] Concurrent local write detected! "
                              "[DISCARD L] new: %llus +%u; "
                              "pending: %llus +%u\n",
                              current->comm, current->pid,
                              (unsigned long long)sector, size,
                              (unsigned long long)i->sector, i->size);
                        goto out_conflict;
                }
        }

        if (mdev->ee_hash_s) {
                /* now, check for overlapping requests with remote origin */
                BUG_ON(mdev->ee_hash == NULL);
#undef OVERLAPS
#define OVERLAPS overlaps(e->sector, e->size, sector, size)
                slot = ee_hash_slot(mdev, sector);
                hlist_for_each_entry(e, n, slot, colision) {
                        if (OVERLAPS) {
                                dev_alert(DEV, "%s[%u] Concurrent remote write detected!"
                                      " [DISCARD L] new: %llus +%u; "
                                      "pending: %llus +%u\n",
                                      current->comm, current->pid,
                                      (unsigned long long)sector, size,
                                      (unsigned long long)e->sector, e->size);
                                goto out_conflict;
                        }
                }
        }
#undef OVERLAPS

out_no_conflict:
        /* this is like it should be, and what we expected.
         * our users do behave after all... */
        put_net_conf(mdev);
        return 0;

out_conflict:
        put_net_conf(mdev);
        return 1;
}

/* obviously this could be coded as many single functions
 * instead of one huge switch,
 * or by putting the code directly in the respective locations
 * (as it has been before).
 *
 * but having it this way
 *  enforces that it is all in this one place, where it is easier to audit,
 *  it makes it obvious that whatever "event" "happens" to a request should
 *  happen "atomically" within the req_lock,
 *  and it enforces that we have to think in a very structured manner
 *  about the "events" that may happen to a request during its life time ...
 */
int __req_mod(struct drbd_request *req, enum drbd_req_event what,
                struct bio_and_error *m)
{
        struct drbd_conf *mdev = req->mdev;
        int rv = 0;
        m->bio = NULL;

        switch (what) {
        default:
                dev_err(DEV, "LOGIC BUG in %s:%u\n", __FILE__ , __LINE__);
                break;

        /* does not happen...
         * initialization done in drbd_req_new
        case created:
                break;
                */

        case to_be_send: /* via network */
                /* reached via drbd_make_request_common
                 * and from w_read_retry_remote */
                D_ASSERT(!(req->rq_state & RQ_NET_MASK));
                req->rq_state |= RQ_NET_PENDING;
                inc_ap_pending(mdev);
                break;

        case to_be_submitted: /* locally */
                /* reached via drbd_make_request_common */
                D_ASSERT(!(req->rq_state & RQ_LOCAL_MASK));
                req->rq_state |= RQ_LOCAL_PENDING;
                break;

        case completed_ok:
                if (bio_data_dir(req->master_bio) == WRITE)
                        mdev->writ_cnt += req->size>>9;
                else
                        mdev->read_cnt += req->size>>9;

                req->rq_state |= (RQ_LOCAL_COMPLETED|RQ_LOCAL_OK);
                req->rq_state &= ~RQ_LOCAL_PENDING;

                _req_may_be_done_not_susp(req, m);
                put_ldev(mdev);
                break;

        case write_completed_with_error:
                req->rq_state |= RQ_LOCAL_COMPLETED;
                req->rq_state &= ~RQ_LOCAL_PENDING;

                __drbd_chk_io_error(mdev, FALSE);
                _req_may_be_done_not_susp(req, m);
                put_ldev(mdev);
                break;

        case read_ahead_completed_with_error:
                /* it is legal to fail READA */
                req->rq_state |= RQ_LOCAL_COMPLETED;
                req->rq_state &= ~RQ_LOCAL_PENDING;
                _req_may_be_done_not_susp(req, m);
                put_ldev(mdev);
                break;

        case read_completed_with_error:
                drbd_set_out_of_sync(mdev, req->sector, req->size);

                req->rq_state |= RQ_LOCAL_COMPLETED;
                req->rq_state &= ~RQ_LOCAL_PENDING;

                D_ASSERT(!(req->rq_state & RQ_NET_MASK));

                __drbd_chk_io_error(mdev, FALSE);
                put_ldev(mdev);

                /* no point in retrying if there is no good remote data,
                 * or we have no connection. */
                if (mdev->state.pdsk != D_UP_TO_DATE) {
                        _req_may_be_done_not_susp(req, m);
                        break;
                }

                /* _req_mod(req,to_be_send); oops, recursion... */
                req->rq_state |= RQ_NET_PENDING;
                inc_ap_pending(mdev);
                /* fall through: _req_mod(req,queue_for_net_read); */

        case queue_for_net_read:
                /* READ or READA, and
                 * no local disk,
                 * or target area marked as invalid,
                 * or just got an io-error. */
                /* from drbd_make_request_common
                 * or from bio_endio during read io-error recovery */

                /* so we can verify the handle in the answer packet
                 * corresponding hlist_del is in _req_may_be_done() */
                hlist_add_head(&req->colision, ar_hash_slot(mdev, req->sector));

                set_bit(UNPLUG_REMOTE, &mdev->flags);

                D_ASSERT(req->rq_state & RQ_NET_PENDING);
                req->rq_state |= RQ_NET_QUEUED;
                req->w.cb = (req->rq_state & RQ_LOCAL_MASK)
                        ? w_read_retry_remote
                        : w_send_read_req;
                drbd_queue_work(&mdev->data.work, &req->w);
                break;

        case queue_for_net_write:
                /* assert something? */
                /* from drbd_make_request_common only */

                hlist_add_head(&req->colision, tl_hash_slot(mdev, req->sector));
                /* corresponding hlist_del is in _req_may_be_done() */

                /* NOTE
                 * In case the req ended up on the transfer log before being
                 * queued on the worker, it could lead to this request being
                 * missed during cleanup after connection loss.
                 * So we have to do both operations here,
                 * within the same lock that protects the transfer log.
                 *
                 * _req_add_to_epoch(req); this has to be after the
                 * _maybe_start_new_epoch(req); which happened in
                 * drbd_make_request_common, because we now may set the bit
                 * again ourselves to close the current epoch.
                 *
                 * Add req to the (now) current epoch (barrier). */

                /* otherwise we may lose an unplug, which may cause some remote
                 * io-scheduler timeout to expire, increasing maximum latency,
                 * hurting performance. */
                set_bit(UNPLUG_REMOTE, &mdev->flags);

                /* see drbd_make_request_common,
                 * just after it grabs the req_lock */
                D_ASSERT(test_bit(CREATE_BARRIER, &mdev->flags) == 0);

                req->epoch = mdev->newest_tle->br_number;

                /* increment size of current epoch */
                mdev->newest_tle->n_writes++;

                /* queue work item to send data */
                D_ASSERT(req->rq_state & RQ_NET_PENDING);
                req->rq_state |= RQ_NET_QUEUED;
                req->w.cb =  w_send_dblock;
                drbd_queue_work(&mdev->data.work, &req->w);

                /* close the epoch, in case it outgrew the limit */
                if (mdev->newest_tle->n_writes >= mdev->net_conf->max_epoch_size)
                        queue_barrier(mdev);

                break;

        case send_canceled:
                /* treat it the same */
        case send_failed:
                /* real cleanup will be done from tl_clear.  just update flags
                 * so it is no longer marked as on the worker queue */
                req->rq_state &= ~RQ_NET_QUEUED;
                /* if we did it right, tl_clear should be scheduled only after
                 * this, so this should not be necessary! */
                _req_may_be_done_not_susp(req, m);
                break;

        case handed_over_to_network:
                /* assert something? */
                if (bio_data_dir(req->master_bio) == WRITE &&
                    mdev->net_conf->wire_protocol == DRBD_PROT_A) {
                        /* this is what is dangerous about protocol A:
                         * pretend it was successfully written on the peer. */
                        if (req->rq_state & RQ_NET_PENDING) {
                                dec_ap_pending(mdev);
                                req->rq_state &= ~RQ_NET_PENDING;
                                req->rq_state |= RQ_NET_OK;
                        } /* else: neg-ack was faster... */
                        /* it is still not yet RQ_NET_DONE until the
                         * corresponding epoch barrier got acked as well,
                         * so we know what to dirty on connection loss */
                }
                req->rq_state &= ~RQ_NET_QUEUED;
                req->rq_state |= RQ_NET_SENT;
                /* because _drbd_send_zc_bio could sleep, and may want to
                 * dereference the bio even after the "write_acked_by_peer" and
                 * "completed_ok" events came in, once we return from
                 * _drbd_send_zc_bio (drbd_send_dblock), we have to check
                 * whether it is done already, and end it.  */
                _req_may_be_done_not_susp(req, m);
                break;

        case read_retry_remote_canceled:
                req->rq_state &= ~RQ_NET_QUEUED;
                /* fall through, in case we raced with drbd_disconnect */
        case connection_lost_while_pending:
                /* transfer log cleanup after connection loss */
                /* assert something? */
                if (req->rq_state & RQ_NET_PENDING)
                        dec_ap_pending(mdev);
                req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);
                req->rq_state |= RQ_NET_DONE;
                /* if it is still queued, we may not complete it here.
                 * it will be canceled soon. */
                if (!(req->rq_state & RQ_NET_QUEUED))
                        _req_may_be_done(req, m); /* Allowed while state.susp */
                break;

        case write_acked_by_peer_and_sis:
                req->rq_state |= RQ_NET_SIS;
        case conflict_discarded_by_peer:
                /* for discarded conflicting writes of multiple primaries,
                 * there is no need to keep anything in the tl, potential
                 * node crashes are covered by the activity log. */
                if (what == conflict_discarded_by_peer)
                        dev_alert(DEV, "Got DiscardAck packet %llus +%u!"
                              " DRBD is not a random data generator!\n",
                              (unsigned long long)req->sector, req->size);
                req->rq_state |= RQ_NET_DONE;
                /* fall through */
        case write_acked_by_peer:
                /* protocol C; successfully written on peer.
                 * Nothing to do here.
                 * We want to keep the tl in place for all protocols, to cater
                 * for volatile write-back caches on lower level devices.
                 *
                 * A barrier request is expected to have forced all prior
                 * requests onto stable storage, so completion of a barrier
                 * request could set NET_DONE right here, and not wait for the
                 * P_BARRIER_ACK, but that is an unnecessary optimization. */

                /* this makes it effectively the same as for: */
        case recv_acked_by_peer:
                /* protocol B; pretends to be successfully written on peer.
                 * see also notes above in handed_over_to_network about
                 * protocol != C */
                req->rq_state |= RQ_NET_OK;
                D_ASSERT(req->rq_state & RQ_NET_PENDING);
                dec_ap_pending(mdev);
                req->rq_state &= ~RQ_NET_PENDING;
                _req_may_be_done_not_susp(req, m);
                break;

        case neg_acked:
                /* assert something? */
                if (req->rq_state & RQ_NET_PENDING)
                        dec_ap_pending(mdev);
                req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);

                req->rq_state |= RQ_NET_DONE;
                _req_may_be_done_not_susp(req, m);
                /* else: done by handed_over_to_network */
                break;

        case fail_frozen_disk_io:
                if (!(req->rq_state & RQ_LOCAL_COMPLETED))
                        break;

                _req_may_be_done(req, m); /* Allowed while state.susp */
                break;

        case restart_frozen_disk_io:
                if (!(req->rq_state & RQ_LOCAL_COMPLETED))
                        break;

                req->rq_state &= ~RQ_LOCAL_COMPLETED;

                rv = MR_READ;
                if (bio_data_dir(req->master_bio) == WRITE)
                        rv = MR_WRITE;

                get_ldev(mdev);
                req->w.cb = w_restart_disk_io;
                drbd_queue_work(&mdev->data.work, &req->w);
                break;

        case resend:
                /* If RQ_NET_OK is already set, we got a P_WRITE_ACK or P_RECV_ACK
                   before the connection loss (B&C only); only P_BARRIER_ACK was missing.
                   Trowing them out of the TL here by pretending we got a BARRIER_ACK
                   We ensure that the peer was not rebooted */
                if (!(req->rq_state & RQ_NET_OK)) {
                        if (req->w.cb) {
                                drbd_queue_work(&mdev->data.work, &req->w);
                                rv = req->rq_state & RQ_WRITE ? MR_WRITE : MR_READ;
                        }
                        break;
                }
                /* else, fall through to barrier_acked */

        case barrier_acked:
                if (!(req->rq_state & RQ_WRITE))
                        break;

                if (req->rq_state & RQ_NET_PENDING) {
                        /* barrier came in before all requests have been acked.
                         * this is bad, because if the connection is lost now,
                         * we won't be able to clean them up... */
                        dev_err(DEV, "FIXME (barrier_acked but pending)\n");
                        list_move(&req->tl_requests, &mdev->out_of_sequence_requests);
                }
                D_ASSERT(req->rq_state & RQ_NET_SENT);
                req->rq_state |= RQ_NET_DONE;
                _req_may_be_done(req, m); /* Allowed while state.susp */
                break;

        case data_received:
                D_ASSERT(req->rq_state & RQ_NET_PENDING);
                dec_ap_pending(mdev);
                req->rq_state &= ~RQ_NET_PENDING;
                req->rq_state |= (RQ_NET_OK|RQ_NET_DONE);
                _req_may_be_done_not_susp(req, m);
                break;
        };

        return rv;
}

/* we may do a local read if:
 * - we are consistent (of course),
 * - or we are generally inconsistent,
 *   BUT we are still/already IN SYNC for this area.
 *   since size may be bigger than BM_BLOCK_SIZE,
 *   we may need to check several bits.
 */
static int drbd_may_do_local_read(struct drbd_conf *mdev, sector_t sector, int size)
{
        unsigned long sbnr, ebnr;
        sector_t esector, nr_sectors;

        if (mdev->state.disk == D_UP_TO_DATE)
                return 1;
        if (mdev->state.disk >= D_OUTDATED)
                return 0;
        if (mdev->state.disk <  D_INCONSISTENT)
                return 0;
        /* state.disk == D_INCONSISTENT   We will have a look at the BitMap */
        nr_sectors = drbd_get_capacity(mdev->this_bdev);
        esector = sector + (size >> 9) - 1;

        D_ASSERT(sector  < nr_sectors);
        D_ASSERT(esector < nr_sectors);

        sbnr = BM_SECT_TO_BIT(sector);
        ebnr = BM_SECT_TO_BIT(esector);

        return 0 == drbd_bm_count_bits(mdev, sbnr, ebnr);
}

static int drbd_make_request_common(struct drbd_conf *mdev, struct bio *bio)
{
        const int rw = bio_rw(bio);
        const int size = bio->bi_size;
        const sector_t sector = bio->bi_sector;
        struct drbd_tl_epoch *b = NULL;
        struct drbd_request *req;
        int local, remote;
        int err = -EIO;
        int ret = 0;

        /* allocate outside of all locks; */
        req = drbd_req_new(mdev, bio);
        if (!req) {
                dec_ap_bio(mdev);
                /* only pass the error to the upper layers.
                 * if user cannot handle io errors, that's not our business. */
                dev_err(DEV, "could not kmalloc() req\n");
                bio_endio(bio, -ENOMEM);
                return 0;
        }

        local = get_ldev(mdev);
        if (!local) {
                bio_put(req->private_bio); /* or we get a bio leak */
                req->private_bio = NULL;
        }
        if (rw == WRITE) {
                remote = 1;
        } else {
                /* READ || READA */
                if (local) {
                        if (!drbd_may_do_local_read(mdev, sector, size)) {
                                /* we could kick the syncer to
                                 * sync this extent asap, wait for
                                 * it, then continue locally.
                                 * Or just issue the request remotely.
                                 */
                                local = 0;
                                bio_put(req->private_bio);
                                req->private_bio = NULL;
                                put_ldev(mdev);
                        }
                }
                remote = !local && mdev->state.pdsk >= D_UP_TO_DATE;
        }

        /* If we have a disk, but a READA request is mapped to remote,
         * we are R_PRIMARY, D_INCONSISTENT, SyncTarget.
         * Just fail that READA request right here.
         *
         * THINK: maybe fail all READA when not local?
         *        or make this configurable...
         *        if network is slow, READA won't do any good.
         */
        if (rw == READA && mdev->state.disk >= D_INCONSISTENT && !local) {
                err = -EWOULDBLOCK;
                goto fail_and_free_req;
        }

        /* For WRITES going to the local disk, grab a reference on the target
         * extent.  This waits for any resync activity in the corresponding
         * resync extent to finish, and, if necessary, pulls in the target
         * extent into the activity log, which involves further disk io because
         * of transactional on-disk meta data updates. */
        if (rw == WRITE && local)
                drbd_al_begin_io(mdev, sector);

        remote = remote && (mdev->state.pdsk == D_UP_TO_DATE ||
                            (mdev->state.pdsk == D_INCONSISTENT &&
                             mdev->state.conn >= C_CONNECTED));

        if (!(local || remote) && !mdev->state.susp) {
                dev_err(DEV, "IO ERROR: neither local nor remote disk\n");
                goto fail_free_complete;
        }

        /* For WRITE request, we have to make sure that we have an
         * unused_spare_tle, in case we need to start a new epoch.
         * I try to be smart and avoid to pre-allocate always "just in case",
         * but there is a race between testing the bit and pointer outside the
         * spinlock, and grabbing the spinlock.
         * if we lost that race, we retry.  */
        if (rw == WRITE && remote &&
            mdev->unused_spare_tle == NULL &&
            test_bit(CREATE_BARRIER, &mdev->flags)) {
allocate_barrier:
                b = kmalloc(sizeof(struct drbd_tl_epoch), GFP_NOIO);
                if (!b) {
                        dev_err(DEV, "Failed to alloc barrier.\n");
                        err = -ENOMEM;
                        goto fail_free_complete;
                }
        }

        /* GOOD, everything prepared, grab the spin_lock */
        spin_lock_irq(&mdev->req_lock);

        if (mdev->state.susp) {
                /* If we got suspended, use the retry mechanism of
                   generic_make_request() to restart processing of this
                   bio. In the next call to drbd_make_request_26
                   we sleep in inc_ap_bio() */
                ret = 1;
                spin_unlock_irq(&mdev->req_lock);
                goto fail_free_complete;
        }

        if (remote) {
                remote = (mdev->state.pdsk == D_UP_TO_DATE ||
                            (mdev->state.pdsk == D_INCONSISTENT &&
                             mdev->state.conn >= C_CONNECTED));
                if (!remote)
                        dev_warn(DEV, "lost connection while grabbing the req_lock!\n");
                if (!(local || remote)) {
                        dev_err(DEV, "IO ERROR: neither local nor remote disk\n");
                        spin_unlock_irq(&mdev->req_lock);
                        goto fail_free_complete;
                }
        }

        if (b && mdev->unused_spare_tle == NULL) {
                mdev->unused_spare_tle = b;
                b = NULL;
        }
        if (rw == WRITE && remote &&
            mdev->unused_spare_tle == NULL &&
            test_bit(CREATE_BARRIER, &mdev->flags)) {
                /* someone closed the current epoch
                 * while we were grabbing the spinlock */
                spin_unlock_irq(&mdev->req_lock);
                goto allocate_barrier;
        }


        /* Update disk stats */
        _drbd_start_io_acct(mdev, req, bio);

        /* _maybe_start_new_epoch(mdev);
         * If we need to generate a write barrier packet, we have to add the
         * new epoch (barrier) object, and queue the barrier packet for sending,
         * and queue the req's data after it _within the same lock_, otherwise
         * we have race conditions were the reorder domains could be mixed up.
         *
         * Even read requests may start a new epoch and queue the corresponding
         * barrier packet.  To get the write ordering right, we only have to
         * make sure that, if this is a write request and it triggered a
         * barrier packet, this request is queued within the same spinlock. */
        if (remote && mdev->unused_spare_tle &&
            test_and_clear_bit(CREATE_BARRIER, &mdev->flags)) {
                _tl_add_barrier(mdev, mdev->unused_spare_tle);
                mdev->unused_spare_tle = NULL;
        } else {
                D_ASSERT(!(remote && rw == WRITE &&
                           test_bit(CREATE_BARRIER, &mdev->flags)));
        }

        /* NOTE
         * Actually, 'local' may be wrong here already, since we may have failed
         * to write to the meta data, and may become wrong anytime because of
         * local io-error for some other request, which would lead to us
         * "detaching" the local disk.
         *
         * 'remote' may become wrong any time because the network could fail.
         *
         * This is a harmless race condition, though, since it is handled
         * correctly at the appropriate places; so it just defers the failure
         * of the respective operation.
         */

        /* mark them early for readability.
         * this just sets some state flags. */
        if (remote)
                _req_mod(req, to_be_send);
        if (local)
                _req_mod(req, to_be_submitted);

        /* check this request on the collision detection hash tables.
         * if we have a conflict, just complete it here.
         * THINK do we want to check reads, too? (I don't think so...) */
        if (rw == WRITE && _req_conflicts(req)) {
                /* this is a conflicting request.
                 * even though it may have been only _partially_
                 * overlapping with one of the currently pending requests,
                 * without even submitting or sending it, we will
                 * pretend that it was successfully served right now.
                 */
                if (local) {
                        bio_put(req->private_bio);
                        req->private_bio = NULL;
                        drbd_al_complete_io(mdev, req->sector);
                        put_ldev(mdev);
                        local = 0;
                }
                if (remote)
                        dec_ap_pending(mdev);
                _drbd_end_io_acct(mdev, req);
                /* THINK: do we want to fail it (-EIO), or pretend success? */
                bio_endio(req->master_bio, 0);
                req->master_bio = NULL;
                dec_ap_bio(mdev);
                drbd_req_free(req);
                remote = 0;
        }


        list_add_tail(&req->tl_requests, &mdev->newest_tle->requests);

        /* NOTE remote first: to get the concurrent write detection right,
         * we must register the request before start of local IO.  */
        if (remote) {
                /* either WRITE and C_CONNECTED,
                 * or READ, and no local disk,
                 * or READ, but not in sync.
                 */
                _req_mod(req, (rw == WRITE)
                                ? queue_for_net_write
                                : queue_for_net_read);
        }
        spin_unlock_irq(&mdev->req_lock);
        kfree(b); /* if someone else has beaten us to it... */

        if (local) {
                req->private_bio->bi_bdev = mdev->ldev->backing_bdev;

                if (FAULT_ACTIVE(mdev, rw == WRITE ? DRBD_FAULT_DT_WR
                                     : rw == READ  ? DRBD_FAULT_DT_RD
                                     :               DRBD_FAULT_DT_RA))
                        bio_endio(req->private_bio, -EIO);
                else
                        generic_make_request(req->private_bio);
        }

        /* we need to plug ALWAYS since we possibly need to kick lo_dev.
         * we plug after submit, so we won't miss an unplug event */
        drbd_plug_device(mdev);

        return 0;

fail_free_complete:
        if (rw == WRITE && local)
                drbd_al_complete_io(mdev, sector);
fail_and_free_req:
        if (local) {
                bio_put(req->private_bio);
                req->private_bio = NULL;
                put_ldev(mdev);
        }
        if (!ret)
                bio_endio(bio, err);

        drbd_req_free(req);
        dec_ap_bio(mdev);
        kfree(b);

        return ret;
}

/* helper function for drbd_make_request
 * if we can determine just by the mdev (state) that this request will fail,
 * return 1
 * otherwise return 0
 */
static int drbd_fail_request_early(struct drbd_conf *mdev, int is_write)
{
        if (mdev->state.role != R_PRIMARY &&
                (!allow_oos || is_write)) {
                if (__ratelimit(&drbd_ratelimit_state)) {
                        dev_err(DEV, "Process %s[%u] tried to %s; "
                            "since we are not in Primary state, "
                            "we cannot allow this\n",
                            current->comm, current->pid,
                            is_write ? "WRITE" : "READ");
                }
                return 1;
        }

        return 0;
}

int drbd_make_request_26(struct request_queue *q, struct bio *bio)
{
        unsigned int s_enr, e_enr;
        struct drbd_conf *mdev = (struct drbd_conf *) q->queuedata;

        if (drbd_fail_request_early(mdev, bio_data_dir(bio) & WRITE)) {
                bio_endio(bio, -EPERM);
                return 0;
        }

        /* Reject barrier requests if we know the underlying device does
         * not support them.
         * XXX: Need to get this info from peer as well some how so we
         * XXX: reject if EITHER side/data/metadata area does not support them.
         *
         * because of those XXX, this is not yet enabled,
         * i.e. in drbd_init_set_defaults we set the NO_BARRIER_SUPP bit.
         */
        if (unlikely(bio->bi_rw & REQ_HARDBARRIER) && test_bit(NO_BARRIER_SUPP, &mdev->flags)) {
                /* dev_warn(DEV, "Rejecting barrier request as underlying device does not support\n"); */
                bio_endio(bio, -EOPNOTSUPP);
                return 0;
        }

        /*
         * what we "blindly" assume:
         */
        D_ASSERT(bio->bi_size > 0);
        D_ASSERT((bio->bi_size & 0x1ff) == 0);
        D_ASSERT(bio->bi_idx == 0);

        /* to make some things easier, force alignment of requests within the
         * granularity of our hash tables */
        s_enr = bio->bi_sector >> HT_SHIFT;
        e_enr = (bio->bi_sector+(bio->bi_size>>9)-1) >> HT_SHIFT;

        if (likely(s_enr == e_enr)) {
                inc_ap_bio(mdev, 1);
                return drbd_make_request_common(mdev, bio);
        }

        /* can this bio be split generically?
         * Maybe add our own split-arbitrary-bios function. */
        if (bio->bi_vcnt != 1 || bio->bi_idx != 0 || bio->bi_size > DRBD_MAX_SEGMENT_SIZE) {
                /* rather error out here than BUG in bio_split */
                dev_err(DEV, "bio would need to, but cannot, be split: "
                    "(vcnt=%u,idx=%u,size=%u,sector=%llu)\n",
                    bio->bi_vcnt, bio->bi_idx, bio->bi_size,
                    (unsigned long long)bio->bi_sector);
                bio_endio(bio, -EINVAL);
        } else {
                /* This bio crosses some boundary, so we have to split it. */
                struct bio_pair *bp;
                /* works for the "do not cross hash slot boundaries" case
                 * e.g. sector 262269, size 4096
                 * s_enr = 262269 >> 6 = 4097
                 * e_enr = (262269+8-1) >> 6 = 4098
                 * HT_SHIFT = 6
                 * sps = 64, mask = 63
                 * first_sectors = 64 - (262269 & 63) = 3
                 */
                const sector_t sect = bio->bi_sector;
                const int sps = 1 << HT_SHIFT; /* sectors per slot */
                const int mask = sps - 1;
                const sector_t first_sectors = sps - (sect & mask);
                bp = bio_split(bio,
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,28)
                                bio_split_pool,
#endif
                                first_sectors);

                /* we need to get a "reference count" (ap_bio_cnt)
                 * to avoid races with the disconnect/reconnect/suspend code.
                 * In case we need to split the bio here, we need to get three references
                 * atomically, otherwise we might deadlock when trying to submit the
                 * second one! */
                inc_ap_bio(mdev, 3);

                D_ASSERT(e_enr == s_enr + 1);

                while (drbd_make_request_common(mdev, &bp->bio1))
                        inc_ap_bio(mdev, 1);

                while (drbd_make_request_common(mdev, &bp->bio2))
                        inc_ap_bio(mdev, 1);

                dec_ap_bio(mdev);

                bio_pair_release(bp);
        }
        return 0;
}

/* This is called by bio_add_page().  With this function we reduce
 * the number of BIOs that span over multiple DRBD_MAX_SEGMENT_SIZEs
 * units (was AL_EXTENTs).
 *
 * we do the calculation within the lower 32bit of the byte offsets,
 * since we don't care for actual offset, but only check whether it
 * would cross "activity log extent" boundaries.
 *
 * As long as the BIO is empty we have to allow at least one bvec,
 * regardless of size and offset.  so the resulting bio may still
 * cross extent boundaries.  those are dealt with (bio_split) in
 * drbd_make_request_26.
 */
int drbd_merge_bvec(struct request_queue *q, struct bvec_merge_data *bvm, struct bio_vec *bvec)
{
        struct drbd_conf *mdev = (struct drbd_conf *) q->queuedata;
        unsigned int bio_offset =
                (unsigned int)bvm->bi_sector << 9; /* 32 bit */
        unsigned int bio_size = bvm->bi_size;
        int limit, backing_limit;

        limit = DRBD_MAX_SEGMENT_SIZE
              - ((bio_offset & (DRBD_MAX_SEGMENT_SIZE-1)) + bio_size);
        if (limit < 0)
                limit = 0;
        if (bio_size == 0) {
                if (limit <= bvec->bv_len)
                        limit = bvec->bv_len;
        } else if (limit && get_ldev(mdev)) {
                struct request_queue * const b =
                        mdev->ldev->backing_bdev->bd_disk->queue;
                if (b->merge_bvec_fn) {
                        backing_limit = b->merge_bvec_fn(b, bvm, bvec);
                        limit = min(limit, backing_limit);
                }
                put_ldev(mdev);
        }
        return limit;
}