[PATCH 04/15] drbd: request state processing

[Philipp Reisner]

Describes the states an IO request issued to DRBD can be in. As well as
all the state transitions that a request can go through.

In here is also the make_request() function, we register in the block
layer of the kernel to pass IO requests to DRBD.

Signed-off-by: Philipp Reisner <philipp.reisner@xxxxxxxxxx>
Signed-off-by: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>

---
diff --git a/drivers/block/drbd/drbd_req.h b/drivers/block/drbd/drbd_req.h
new file mode 100644
index 0000000..81393ff
--- /dev/null
+++ b/drivers/block/drbd/drbd_req.h
@@ -0,0 +1,299 @@
+/*
+   drbd_req.h
+
+   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
+
+   Copyright (C) 2006-2008, LINBIT Information Technologies GmbH.
+   Copyright (C) 2006-2008, Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>.
+   Copyright (C) 2006-2008, Philipp Reisner <philipp.reisner@xxxxxxxxxx>.
+
+   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.
+ */
+
+#ifndef _DRBD_REQ_H
+#define _DRBD_REQ_H
+
+#include <linux/autoconf.h>
+#include <linux/module.h>
+
+#include <linux/slab.h>
+#include <linux/drbd.h>
+#include "drbd_int.h"
+#include "drbd_wrappers.h"
+
+/* The request callbacks will be called in irq context by the IDE drivers,
+   and in Softirqs/Tasklets/BH context by the SCSI drivers,
+   and by the receiver and worker in kernel-thread context.
+   Try to get the locking right :) */
+
+/*
+ * Objects of type struct drbd_request do only exist on a R_PRIMARY node, and are
+ * associated with IO requests originating from the block layer above us.
+ *
+ * There are quite a few things that may happen to a drbd request
+ * during its lifetime.
+ *
+ *  It will be created.
+ *  It will be marked with the intention to be
+ *    submitted to local disk and/or
+ *    send via the network.
+ *
+ *  It has to be placed on the transfer log and other housekeeping lists,
+ *  In case we have a network connection.
+ *
+ *  It may be identified as a concurrent (write) request
+ *    and be handled accordingly.
+ *
+ *  It may me handed over to the local disk subsystem.
+ *  It may be completed by the local disk subsystem,
+ *    either sucessfully or with io-error.
+ *  In case it is a READ request, and it failed locally,
+ *    it may be retried remotely.
+ *
+ *  It may be queued for sending.
+ *  It may be handed over to the network stack,
+ *    which may fail.
+ *  It may be acknowledged by the "peer" according to the wire_protocol in use.
+ *    this may be a negative ack.
+ *  It may receive a faked ack when the network connection is lost and the
+ *  transfer log is cleaned up.
+ *  Sending may be canceled due to network connection loss.
+ *  When it finally has outlived its time,
+ *    corresponding dirty bits in the resync-bitmap may be cleared or set,
+ *    it will be destroyed,
+ *    and completion will be signalled to the originator,
+ *      with or without "success".
+ */
+
+enum drbd_req_event {
+	created,
+	to_be_send,
+	to_be_submitted,
+
+	/* XXX yes, now I am inconsistent...
+	 * these two are not "events" but "actions"
+	 * oh, well... */
+	queue_for_net_write,
+	queue_for_net_read,
+
+	send_canceled,
+	send_failed,
+	handed_over_to_network,
+	connection_lost_while_pending,
+	recv_acked_by_peer,
+	write_acked_by_peer,
+	write_acked_by_peer_and_sis, /* and set_in_sync */
+	conflict_discarded_by_peer,
+	neg_acked,
+	barrier_acked, /* in protocol A and B */
+	data_received, /* (remote read) */
+
+	read_completed_with_error,
+	write_completed_with_error,
+	completed_ok,
+	nothing, /* for tracing only */
+};
+
+/* encoding of request states for now.  we don't actually need that many bits.
+ * we don't need to do atomic bit operations either, since most of the time we
+ * need to look at the connection state and/or manipulate some lists at the
+ * same time, so we should hold the request lock anyways.
+ */
+enum drbd_req_state_bits {
+	/* 210
+	 * 000: no local possible
+	 * 001: to be submitted
+	 *    UNUSED, we could map: 011: submitted, completion still pending
+	 * 110: completed ok
+	 * 010: completed with error
+	 */
+	__RQ_LOCAL_PENDING,
+	__RQ_LOCAL_COMPLETED,
+	__RQ_LOCAL_OK,
+
+	/* 76543
+	 * 00000: no network possible
+	 * 00001: to be send
+	 * 00011: to be send, on worker queue
+	 * 00101: sent, expecting recv_ack (B) or write_ack (C)
+	 * 11101: sent,
+	 *        recv_ack (B) or implicit "ack" (A),
+	 *        still waiting for the barrier ack.
+	 *        master_bio may already be completed and invalidated.
+	 * 11100: write_acked (C),
+	 *        data_received (for remote read, any protocol)
+	 *        or finally the barrier ack has arrived (B,A)...
+	 *        request can be freed
+	 * 01100: neg-acked (write, protocol C)
+	 *        or neg-d-acked (read, any protocol)
+	 *        or killed from the transfer log
+	 *        during cleanup after connection loss
+	 *        request can be freed
+	 * 01000: canceled or send failed...
+	 *        request can be freed
+	 */
+
+	/* if "SENT" is not set, yet, this can still fail or be canceled.
+	 * if "SENT" is set already, we still wait for an Ack packet.
+	 * when cleared, the master_bio may be completed.
+	 * in (B,A) the request object may still linger on the transaction log
+	 * until the corresponding barrier ack comes in */
+	__RQ_NET_PENDING,
+
+	/* If it is QUEUED, and it is a WRITE, it is also registered in the
+	 * transfer log. Currently we need this flag to avoid conflicts between
+	 * worker canceling the request and tl_clear_barrier killing it from
+	 * transfer log.  We should restructure the code so this conflict does
+	 * no longer occur. */
+	__RQ_NET_QUEUED,
+
+	/* well, actually only "handed over to the network stack".
+	 *
+	 * TODO can potentially be dropped because of the similar meaning
+	 * of RQ_NET_SENT and ~RQ_NET_QUEUED.
+	 * however it is not exactly the same. before we drop it
+	 * we must ensure that we can tell a request with network part
+	 * from a request without, regardless of what happens to it. */
+	__RQ_NET_SENT,
+
+	/* when set, the request may be freed (if RQ_NET_QUEUED is clear).
+	 * basically this means the corresponding P_BARRIER_ACK was received */
+	__RQ_NET_DONE,
+
+	/* whether or not we know (C) or pretend (B,A) that the write
+	 * was successfully written on the peer.
+	 */
+	__RQ_NET_OK,
+
+	/* peer called drbd_set_in_sync() for this write */
+	__RQ_NET_SIS,
+
+	/* keep this last, its for the RQ_NET_MASK */
+	__RQ_NET_MAX,
+};
+
+#define RQ_LOCAL_PENDING   (1UL << __RQ_LOCAL_PENDING)
+#define RQ_LOCAL_COMPLETED (1UL << __RQ_LOCAL_COMPLETED)
+#define RQ_LOCAL_OK        (1UL << __RQ_LOCAL_OK)
+
+#define RQ_LOCAL_MASK      ((RQ_LOCAL_OK << 1)-1) /* 0x07 */
+
+#define RQ_NET_PENDING     (1UL << __RQ_NET_PENDING)
+#define RQ_NET_QUEUED      (1UL << __RQ_NET_QUEUED)
+#define RQ_NET_SENT        (1UL << __RQ_NET_SENT)
+#define RQ_NET_DONE        (1UL << __RQ_NET_DONE)
+#define RQ_NET_OK          (1UL << __RQ_NET_OK)
+#define RQ_NET_SIS         (1UL << __RQ_NET_SIS)
+
+/* 0x1f8 */
+#define RQ_NET_MASK        (((1UL << __RQ_NET_MAX)-1) & ~RQ_LOCAL_MASK)
+
+/* epoch entries */
+static inline
+struct hlist_head *ee_hash_slot(struct drbd_conf *mdev, sector_t sector)
+{
+	BUG_ON(mdev->ee_hash_s == 0);
+	return mdev->ee_hash +
+		((unsigned int)(sector>>HT_SHIFT) % mdev->ee_hash_s);
+}
+
+/* transfer log (drbd_request objects) */
+static inline
+struct hlist_head *tl_hash_slot(struct drbd_conf *mdev, sector_t sector)
+{
+	BUG_ON(mdev->tl_hash_s == 0);
+	return mdev->tl_hash +
+		((unsigned int)(sector>>HT_SHIFT) % mdev->tl_hash_s);
+}
+
+/* application reads (drbd_request objects) */
+static struct hlist_head *ar_hash_slot(struct drbd_conf *mdev, sector_t sector)
+{
+	return mdev->app_reads_hash
+		+ ((unsigned int)(sector) % APP_R_HSIZE);
+}
+
+/* when we receive the answer for a read request,
+ * verify that we actually know about it */
+static inline struct drbd_request *_ar_id_to_req(struct drbd_conf *mdev,
+	u64 id, sector_t sector)
+{
+	struct hlist_head *slot = ar_hash_slot(mdev, sector);
+	struct hlist_node *n;
+	struct drbd_request *req;
+
+	hlist_for_each_entry(req, n, slot, colision) {
+		if ((unsigned long)req == (unsigned long)id) {
+			D_ASSERT(req->sector == sector);
+			return req;
+		}
+	}
+	return NULL;
+}
+
+static inline struct drbd_request *drbd_req_new(struct drbd_conf *mdev,
+	struct bio *bio_src)
+{
+	struct bio *bio;
+	struct drbd_request *req =
+		mempool_alloc(drbd_request_mempool, GFP_NOIO);
+	if (likely(req)) {
+		bio = bio_clone(bio_src, GFP_NOIO); /* XXX cannot fail?? */
+
+		req->rq_state    = 0;
+		req->mdev        = mdev;
+		req->master_bio  = bio_src;
+		req->private_bio = bio;
+		req->epoch       = 0;
+		req->sector      = bio->bi_sector;
+		req->size        = bio->bi_size;
+		req->start_time  = jiffies;
+		INIT_HLIST_NODE(&req->colision);
+		INIT_LIST_HEAD(&req->tl_requests);
+		INIT_LIST_HEAD(&req->w.list);
+
+		bio->bi_private  = req;
+		bio->bi_end_io   = drbd_endio_pri;
+		bio->bi_next     = NULL;
+	}
+	return req;
+}
+
+static inline void drbd_req_free(struct drbd_request *req)
+{
+	mempool_free(req, drbd_request_mempool);
+}
+
+static inline int overlaps(sector_t s1, int l1, sector_t s2, int l2)
+{
+	return !((s1 + (l1>>9) <= s2) || (s1 >= s2 + (l2>>9)));
+}
+
+/* aparently too large to be inlined...
+ * moved to drbd_req.c */
+extern void _req_may_be_done(struct drbd_request *req, int error);
+extern void _req_mod(struct drbd_request *req,
+		enum drbd_req_event what, int error);
+
+/* If you need it irqsave, do it your self! */
+static inline void req_mod(struct drbd_request *req,
+		enum drbd_req_event what, int error)
+{
+	struct drbd_conf *mdev = req->mdev;
+	spin_lock_irq(&mdev->req_lock);
+	_req_mod(req, what, error);
+	spin_unlock_irq(&mdev->req_lock);
+}
+#endif
diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
new file mode 100644
index 0000000..d2b941c
--- /dev/null
+++ b/drivers/block/drbd/drbd_req.c
@@ -0,0 +1,1130 @@
+/*
+   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@xxxxxxxxxx>.
+   Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>.
+
+   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/autoconf.h>
+#include <linux/module.h>
+
+#include <linux/slab.h>
+#include <linux/drbd.h>
+#include "drbd_int.h"
+#include "drbd_tracing.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_stat_unlock();
+	mdev->vdisk->part0.in_flight++;
+}
+
+/* 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_stat_unlock();
+	mdev->vdisk->part0.in_flight--;
+}
+
+static void _req_is_done(struct drbd_conf *mdev, struct drbd_request *req, const int rw)
+{
+	const unsigned long s = req->rq_state;
+	/* 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) {
+		/* 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
+		 * initialised in drbd_req_new(), so we can list_del() it
+		 * here unconditionally */
+		list_del(&req->tl_requests);
+		/* 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);
+			}
+		}
+	}
+
+	/* if it was a local io error, we want to notify our
+	 * peer about that, and see if we need to
+	 * detach the disk and stuff.
+	 * to avoid allocating some special work
+	 * struct, reuse the request. */
+
+	/* THINK
+	 * why do we do this not when we detect the error,
+	 * but delay it until it is "done", i.e. possibly
+	 * until the next barrier ack? */
+
+	if (rw == WRITE &&
+	    ((s & RQ_LOCAL_MASK) && !(s & RQ_LOCAL_OK))) {
+		if (!(req->w.list.next == LIST_POISON1 ||
+		      list_empty(&req->w.list))) {
+			/* DEBUG ASSERT only; if this triggers, we
+			 * probably corrupt the worker list here */
+			dev_err(DEV, "req->w.list.next = %p\n", req->w.list.next);
+			dev_err(DEV, "req->w.list.prev = %p\n", req->w.list.prev);
+		}
+		req->w.cb = w_io_error;
+		drbd_queue_work(&mdev->data.work, &req->w);
+		/* drbd_req_free() is done in w_io_error */
+	} else {
+		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
+}
+
+static void _complete_master_bio(struct drbd_conf *mdev,
+	struct drbd_request *req, int error)
+{
+	trace_drbd_bio(mdev, "Rq", req->master_bio, 1, req);
+	bio_endio(req->master_bio, error);
+	req->master_bio = NULL;
+	dec_ap_bio(mdev);
+}
+
+void _req_may_be_done(struct drbd_request *req, int error)
+{
+	const unsigned long s = req->rq_state;
+	struct drbd_conf *mdev = req->mdev;
+	int rw;
+
+	trace_drbd_req(req, nothing, "_req_may_be_done");
+
+	/* 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 suceeded.
+		 * 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.
+		 */
+		int ok = (s & RQ_LOCAL_OK) || (s & RQ_NET_OK);
+		rw = bio_data_dir(req->master_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);
+
+		_complete_master_bio(mdev, req,
+				     ok ? 0 : (error ? error : -EIO));
+	} else {
+		/* only WRITE requests can end up here without a master_bio */
+		rw = WRITE;
+	}
+
+	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... */
+}
+
+/*
+ * 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 ...
+ */
+void _req_mod(struct drbd_request *req, enum drbd_req_event what, int error)
+{
+	struct drbd_conf *mdev = req->mdev;
+
+	if (error && (bio_rw(req->master_bio) != READA))
+		dev_err(DEV, "got an _req_mod() errno of %d\n", error);
+
+	trace_drbd_req(req, what, NULL);
+
+	switch (what) {
+	default:
+		dev_err(DEV, "LOGIC BUG in %s:%u\n", __FILE__ , __LINE__);
+		return;
+
+	/* 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->private_bio) == WRITE)
+			mdev->writ_cnt += req->size>>9;
+		else
+			mdev->read_cnt += req->size>>9;
+
+		bio_put(req->private_bio);
+		req->private_bio = NULL;
+
+		req->rq_state |= (RQ_LOCAL_COMPLETED|RQ_LOCAL_OK);
+		req->rq_state &= ~RQ_LOCAL_PENDING;
+
+		_req_may_be_done(req, error);
+		put_ldev(mdev);
+		break;
+
+	case write_completed_with_error:
+		req->rq_state |= RQ_LOCAL_COMPLETED;
+		req->rq_state &= ~RQ_LOCAL_PENDING;
+
+		bio_put(req->private_bio);
+		req->private_bio = NULL;
+		dev_alert(DEV, "Local WRITE failed sec=%llus size=%u\n",
+		      (unsigned long long)req->sector, req->size);
+		/* and now: check how to handle local io error. */
+		__drbd_chk_io_error(mdev, FALSE);
+		_req_may_be_done(req, error);
+		put_ldev(mdev);
+		break;
+
+	case read_completed_with_error:
+		if (bio_rw(req->master_bio) != READA)
+			drbd_set_out_of_sync(mdev, req->sector, req->size);
+
+		req->rq_state |= RQ_LOCAL_COMPLETED;
+		req->rq_state &= ~RQ_LOCAL_PENDING;
+
+		bio_put(req->private_bio);
+		req->private_bio = NULL;
+		if (bio_rw(req->master_bio) == READA) {
+			/* it is legal to fail READA */
+			_req_may_be_done(req, error);
+			put_ldev(mdev);
+			break;
+		}
+		/* else */
+		dev_alert(DEV, "Local READ failed sec=%llus size=%u\n",
+		      (unsigned long long)req->sector, req->size);
+		/* _req_mod(req,to_be_send); oops, recursion... */
+		D_ASSERT(!(req->rq_state & RQ_NET_MASK));
+		req->rq_state |= RQ_NET_PENDING;
+		inc_ap_pending(mdev);
+
+		__drbd_chk_io_error(mdev, FALSE);
+		put_ldev(mdev);
+		/* NOTE: if we have no connection,
+		 * or know the peer has no good data either,
+		 * then we don't actually need to "queue_for_net_read",
+		 * but we do so anyways, since the drbd_io_error()
+		 * and the potential state change to "Diskless"
+		 * needs to be done from process context */
+
+		/* 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); /* why? */
+
+		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). */
+
+		/* 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;
+		list_add_tail(&req->tl_requests,
+				&mdev->newest_tle->requests);
+
+		/* increment size of current epoch */
+		mdev->newest_tle->n_req++;
+
+		/* 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_req >= 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(req, error);
+		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 sucessfully 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(req, error);
+		break;
+
+	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, error);
+		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 primarys,
+		 * there is no need to keep anything in the tl, potential
+		 * node crashes are covered by the activity log. */
+		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 unecessary optimisation. */
+
+		/* this makes it effectively the same as for: */
+	case recv_acked_by_peer:
+		/* protocol B; pretends to be sucessfully 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(req, error);
+		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(req, error);
+		/* else: done by handed_over_to_network */
+		break;
+
+	case barrier_acked:
+		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");
+			trace_drbd_req(req, nothing, "FIXME (barrier_acked but pending)");
+			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, error);
+		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(req, error);
+		break;
+	};
+}
+
+/* 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;
+
+	/* 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, thats not our business. */
+		dev_err(DEV, "could not kmalloc() req\n");
+		bio_endio(bio, -ENOMEM);
+		return 0;
+	}
+
+	trace_drbd_bio(mdev, "Rq", bio, 0, req);
+
+	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)) {
+		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 (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 deferres 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, 0);
+	if (local)
+		_req_mod(req, to_be_submitted, 0);
+
+	/* check this request on the colison 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;
+	}
+
+	/* 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.
+		 */
+		if (rw == WRITE)
+			_req_mod(req, queue_for_net_write, 0);
+		else
+			_req_mod(req, queue_for_net_read, 0);
+	}
+	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;
+
+		trace_drbd_bio(mdev, "Pri", req->private_bio, 0, NULL);
+
+		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);
+	}
+	bio_endio(bio, err);
+	drbd_req_free(req);
+	dec_ap_bio(mdev);
+	kfree(b);
+
+	return 0;
+}
+
+/* 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)
+{
+	/* Unconfigured */
+	if (mdev->state.conn == C_DISCONNECTING &&
+	    mdev->state.disk == D_DISKLESS)
+		return 1;
+
+	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;
+	}
+
+	/*
+	 * Paranoia: we might have been primary, but sync target, or
+	 * even diskless, then lost the connection.
+	 * This should have been handled (panic? suspend?) somehwere
+	 * else. But maybe it was not, so check again here.
+	 * Caution: as long as we do not have a read/write lock on mdev,
+	 * to serialize state changes, this is racy, since we may lose
+	 * the connection *after* we test for the cstate.
+	 */
+	if (mdev->state.disk < D_UP_TO_DATE && mdev->state.pdsk < D_UP_TO_DATE) {
+		if (__ratelimit(&drbd_ratelimit_state))
+			dev_err(DEV, "Sorry, I have no access to good data anymore.\n");
+		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_barrier(bio) && 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 allignment 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 two references
+		 * atomically, otherwise we might deadlock when trying to submit the
+		 * second one! */
+		inc_ap_bio(mdev, 2);
+
+		D_ASSERT(e_enr == s_enr + 1);
+
+		drbd_make_request_common(mdev, &bp->bio1);
+		drbd_make_request_common(mdev, &bp->bio2);
+		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 emtpy 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 && mdev->ldev->dc.use_bmbv) {
+			backing_limit = b->merge_bvec_fn(b, bvm, bvec);
+			limit = min(limit, backing_limit);
+		}
+		put_ldev(mdev);
+	}
+	return limit;
+}
--
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