xfs: Introduce delayed logging core code

The delayed logging code only changes in-memory structures and as
such can be enabled and disabled with a mount option. Add the mount
option and emit a warning that this is an experimental feature that
should not be used in production yet.

We also need infrastructure to track committed items that have not
yet been written to the log. This is what the Committed Item List
(CIL) is for.

The log item also needs to be extended to track the current log
vector, the associated memory buffer and it's location in the Commit
Item List. Extend the log item and log vector structures to enable
this tracking.

To maintain the current log format for transactions with delayed
logging, we need to introduce a checkpoint transaction and a context
for tracking each checkpoint from initiation to transaction
completion.  This includes adding a log ticket for tracking space
log required/used by the context checkpoint.

To track all the changes we need an io vector array per log item,
rather than a single array for the entire transaction. Using the new
log vector structure for this requires two passes - the first to
allocate the log vector structures and chain them together, and the
second to fill them out.  This log vector chain can then be passed
to the CIL for formatting, pinning and insertion into the CIL.

Formatting of the log vector chain is relatively simple - it's just
a loop over the iovecs on each log vector, but it is made slightly
more complex because we re-write the iovec after the copy to point
back at the memory buffer we just copied into.

This code also needs to pin log items. If the log item is not
already tracked in this checkpoint context, then it needs to be
pinned. Otherwise it is already pinned and we don't need to pin it
again.

The only other complexity is calculating the amount of new log space
the formatting has consumed. This needs to be accounted to the
transaction in progress, and the accounting is made more complex
becase we need also to steal space from it for log metadata in the
checkpoint transaction. Calculate all this at insert time and update
all the tickets, counters, etc correctly.

Once we've formatted all the log items in the transaction, attach
the busy extents to the checkpoint context so the busy extents live
until checkpoint completion and can be processed at that point in
time. Transactions can then be freed at this point in time.

Now we need to issue checkpoints - we are tracking the amount of log space
used by the items in the CIL, so we can trigger background checkpoints when the
space usage gets to a certain threshold. Otherwise, checkpoints need ot be
triggered when a log synchronisation point is reached - a log force event.

Because the log write code already handles chained log vectors, writing the
transaction is trivial, too. Construct a transaction header, add it
to the head of the chain and write it into the log, then issue a
commit record write. Then we can release the checkpoint log ticket
and attach the context to the log buffer so it can be called during
Io completion to complete the checkpoint.

We also need to allow for synchronising multiple in-flight
checkpoints. This is needed for two things - the first is to ensure
that checkpoint commit records appear in the log in the correct
sequence order (so they are replayed in the correct order). The
second is so that xfs_log_force_lsn() operates correctly and only
flushes and/or waits for the specific sequence it was provided with.

To do this we need a wait variable and a list tracking the
checkpoint commits in progress. We can walk this list and wait for
the checkpoints to change state or complete easily, an this provides
the necessary synchronisation for correct operation in both cases.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

1 files changed, 659 insertions, 0 deletions

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
new file mode 100644
index 0000000..53abd6b
--- /dev/null
+++ b/fs/xfs/xfs_log_cil.c
@@ -0,0 +1,659 @@
+/*
+ * Copyright (c) 2010 Red Hat, Inc. All Rights Reserved.
+ *
+ * This program 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.
+ *
+ * This program is distributed in the hope that it would 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 this program; if not, write the Free Software Foundation,
+ * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#include "xfs.h"
+#include "xfs_fs.h"
+#include "xfs_types.h"
+#include "xfs_bit.h"
+#include "xfs_log.h"
+#include "xfs_inum.h"
+#include "xfs_trans.h"
+#include "xfs_trans_priv.h"
+#include "xfs_log_priv.h"
+#include "xfs_sb.h"
+#include "xfs_ag.h"
+#include "xfs_dir2.h"
+#include "xfs_dmapi.h"
+#include "xfs_mount.h"
+#include "xfs_error.h"
+#include "xfs_alloc.h"
+
+/*
+ * Perform initial CIL structure initialisation. If the CIL is not
+ * enabled in this filesystem, ensure the log->l_cilp is null so
+ * we can check this conditional to determine if we are doing delayed
+ * logging or not.
+ */
+int
+xlog_cil_init(
+	struct log	*log)
+{
+	struct xfs_cil	*cil;
+	struct xfs_cil_ctx *ctx;
+
+	log->l_cilp = NULL;
+	if (!(log->l_mp->m_flags & XFS_MOUNT_DELAYLOG))
+		return 0;
+
+	cil = kmem_zalloc(sizeof(*cil), KM_SLEEP|KM_MAYFAIL);
+	if (!cil)
+		return ENOMEM;
+
+	ctx = kmem_zalloc(sizeof(*ctx), KM_SLEEP|KM_MAYFAIL);
+	if (!ctx) {
+		kmem_free(cil);
+		return ENOMEM;
+	}
+
+	INIT_LIST_HEAD(&cil->xc_cil);
+	INIT_LIST_HEAD(&cil->xc_committing);
+	spin_lock_init(&cil->xc_cil_lock);
+	init_rwsem(&cil->xc_ctx_lock);
+	sv_init(&cil->xc_commit_wait, SV_DEFAULT, "cilwait");
+
+	INIT_LIST_HEAD(&ctx->committing);
+	INIT_LIST_HEAD(&ctx->busy_extents);
+	ctx->sequence = 1;
+	ctx->cil = cil;
+	cil->xc_ctx = ctx;
+
+	cil->xc_log = log;
+	log->l_cilp = cil;
+	return 0;
+}
+
+void
+xlog_cil_destroy(
+	struct log	*log)
+{
+	if (!log->l_cilp)
+		return;
+
+	if (log->l_cilp->xc_ctx) {
+		if (log->l_cilp->xc_ctx->ticket)
+			xfs_log_ticket_put(log->l_cilp->xc_ctx->ticket);
+		kmem_free(log->l_cilp->xc_ctx);
+	}
+
+	ASSERT(list_empty(&log->l_cilp->xc_cil));
+	kmem_free(log->l_cilp);
+}
+
+/*
+ * Allocate a new ticket. Failing to get a new ticket makes it really hard to
+ * recover, so we don't allow failure here. Also, we allocate in a context that
+ * we don't want to be issuing transactions from, so we need to tell the
+ * allocation code this as well.
+ *
+ * We don't reserve any space for the ticket - we are going to steal whatever
+ * space we require from transactions as they commit. To ensure we reserve all
+ * the space required, we need to set the current reservation of the ticket to
+ * zero so that we know to steal the initial transaction overhead from the
+ * first transaction commit.
+ */
+static struct xlog_ticket *
+xlog_cil_ticket_alloc(
+	struct log	*log)
+{
+	struct xlog_ticket *tic;
+
+	tic = xlog_ticket_alloc(log, 0, 1, XFS_TRANSACTION, 0,
+				KM_SLEEP|KM_NOFS);
+	tic->t_trans_type = XFS_TRANS_CHECKPOINT;
+
+	/*
+	 * set the current reservation to zero so we know to steal the basic
+	 * transaction overhead reservation from the first transaction commit.
+	 */
+	tic->t_curr_res = 0;
+	return tic;
+}
+
+/*
+ * After the first stage of log recovery is done, we know where the head and
+ * tail of the log are. We need this log initialisation done before we can
+ * initialise the first CIL checkpoint context.
+ *
+ * Here we allocate a log ticket to track space usage during a CIL push.  This
+ * ticket is passed to xlog_write() directly so that we don't slowly leak log
+ * space by failing to account for space used by log headers and additional
+ * region headers for split regions.
+ */
+void
+xlog_cil_init_post_recovery(
+	struct log	*log)
+{
+	if (!log->l_cilp)
+		return;
+
+	log->l_cilp->xc_ctx->ticket = xlog_cil_ticket_alloc(log);
+	log->l_cilp->xc_ctx->sequence = 1;
+	log->l_cilp->xc_ctx->commit_lsn = xlog_assign_lsn(log->l_curr_cycle,
+								log->l_curr_block);
+}
+
+/*
+ * Insert the log item into the CIL and calculate the difference in space
+ * consumed by the item. Add the space to the checkpoint ticket and calculate
+ * if the change requires additional log metadata. If it does, take that space
+ * as well. Remove the amount of space we addded to the checkpoint ticket from
+ * the current transaction ticket so that the accounting works out correctly.
+ *
+ * If this is the first time the item is being placed into the CIL in this
+ * context, pin it so it can't be written to disk until the CIL is flushed to
+ * the iclog and the iclog written to disk.
+ */
+static void
+xlog_cil_insert(
+	struct log		*log,
+	struct xlog_ticket	*ticket,
+	struct xfs_log_item	*item,
+	struct xfs_log_vec	*lv)
+{
+	struct xfs_cil		*cil = log->l_cilp;
+	struct xfs_log_vec	*old = lv->lv_item->li_lv;
+	struct xfs_cil_ctx	*ctx = cil->xc_ctx;
+	int			len;
+	int			diff_iovecs;
+	int			iclog_space;
+
+	if (old) {
+		/* existing lv on log item, space used is a delta */
+		ASSERT(!list_empty(&item->li_cil));
+		ASSERT(old->lv_buf && old->lv_buf_len && old->lv_niovecs);
+
+		len = lv->lv_buf_len - old->lv_buf_len;
+		diff_iovecs = lv->lv_niovecs - old->lv_niovecs;
+		kmem_free(old->lv_buf);
+		kmem_free(old);
+	} else {
+		/* new lv, must pin the log item */
+		ASSERT(!lv->lv_item->li_lv);
+		ASSERT(list_empty(&item->li_cil));
+
+		len = lv->lv_buf_len;
+		diff_iovecs = lv->lv_niovecs;
+		IOP_PIN(lv->lv_item);
+
+	}
+	len += diff_iovecs * sizeof(xlog_op_header_t);
+
+	/* attach new log vector to log item */
+	lv->lv_item->li_lv = lv;
+
+	spin_lock(&cil->xc_cil_lock);
+	list_move_tail(&item->li_cil, &cil->xc_cil);
+	ctx->nvecs += diff_iovecs;
+
+	/*
+	 * Now transfer enough transaction reservation to the context ticket
+	 * for the checkpoint. The context ticket is special - the unit
+	 * reservation has to grow as well as the current reservation as we
+	 * steal from tickets so we can correctly determine the space used
+	 * during the transaction commit.
+	 */
+	if (ctx->ticket->t_curr_res == 0) {
+		/* first commit in checkpoint, steal the header reservation */
+		ASSERT(ticket->t_curr_res >= ctx->ticket->t_unit_res + len);
+		ctx->ticket->t_curr_res = ctx->ticket->t_unit_res;
+		ticket->t_curr_res -= ctx->ticket->t_unit_res;
+	}
+
+	/* do we need space for more log record headers? */
+	iclog_space = log->l_iclog_size - log->l_iclog_hsize;
+	if (len > 0 && (ctx->space_used / iclog_space !=
+				(ctx->space_used + len) / iclog_space)) {
+		int hdrs;
+
+		hdrs = (len + iclog_space - 1) / iclog_space;
+		/* need to take into account split region headers, too */
+		hdrs *= log->l_iclog_hsize + sizeof(struct xlog_op_header);
+		ctx->ticket->t_unit_res += hdrs;
+		ctx->ticket->t_curr_res += hdrs;
+		ticket->t_curr_res -= hdrs;
+		ASSERT(ticket->t_curr_res >= len);
+	}
+	ticket->t_curr_res -= len;
+	ctx->space_used += len;
+
+	spin_unlock(&cil->xc_cil_lock);
+}
+
+/*
+ * Format log item into a flat buffers
+ *
+ * For delayed logging, we need to hold a formatted buffer containing all the
+ * changes on the log item. This enables us to relog the item in memory and
+ * write it out asynchronously without needing to relock the object that was
+ * modified at the time it gets written into the iclog.
+ *
+ * This function builds a vector for the changes in each log item in the
+ * transaction. It then works out the length of the buffer needed for each log
+ * item, allocates them and formats the vector for the item into the buffer.
+ * The buffer is then attached to the log item are then inserted into the
+ * Committed Item List for tracking until the next checkpoint is written out.
+ *
+ * We don't set up region headers during this process; we simply copy the
+ * regions into the flat buffer. We can do this because we still have to do a
+ * formatting step to write the regions into the iclog buffer.  Writing the
+ * ophdrs during the iclog write means that we can support splitting large
+ * regions across iclog boundares without needing a change in the format of the
+ * item/region encapsulation.
+ *
+ * Hence what we need to do now is change the rewrite the vector array to point
+ * to the copied region inside the buffer we just allocated. This allows us to
+ * format the regions into the iclog as though they are being formatted
+ * directly out of the objects themselves.
+ */
+static void
+xlog_cil_format_items(
+	struct log		*log,
+	struct xfs_log_vec	*log_vector,
+	struct xlog_ticket	*ticket,
+	xfs_lsn_t		*start_lsn)
+{
+	struct xfs_log_vec *lv;
+
+	if (start_lsn)
+		*start_lsn = log->l_cilp->xc_ctx->sequence;
+
+	ASSERT(log_vector);
+	for (lv = log_vector; lv; lv = lv->lv_next) {
+		void	*ptr;
+		int	index;
+		int	len = 0;
+
+		/* build the vector array and calculate it's length */
+		IOP_FORMAT(lv->lv_item, lv->lv_iovecp);
+		for (index = 0; index < lv->lv_niovecs; index++)
+			len += lv->lv_iovecp[index].i_len;
+
+		lv->lv_buf_len = len;
+		lv->lv_buf = kmem_zalloc(lv->lv_buf_len, KM_SLEEP|KM_NOFS);
+		ptr = lv->lv_buf;
+
+		for (index = 0; index < lv->lv_niovecs; index++) {
+			struct xfs_log_iovec *vec = &lv->lv_iovecp[index];
+
+			memcpy(ptr, vec->i_addr, vec->i_len);
+			vec->i_addr = ptr;
+			ptr += vec->i_len;
+		}
+		ASSERT(ptr == lv->lv_buf + lv->lv_buf_len);
+
+		xlog_cil_insert(log, ticket, lv->lv_item, lv);
+	}
+}
+
+static void
+xlog_cil_free_logvec(
+	struct xfs_log_vec	*log_vector)
+{
+	struct xfs_log_vec	*lv;
+
+	for (lv = log_vector; lv; ) {
+		struct xfs_log_vec *next = lv->lv_next;
+		kmem_free(lv->lv_buf);
+		kmem_free(lv);
+		lv = next;
+	}
+}
+
+/*
+ * Commit a transaction with the given vector to the Committed Item List.
+ *
+ * To do this, we need to format the item, pin it in memory if required and
+ * account for the space used by the transaction. Once we have done that we
+ * need to release the unused reservation for the transaction, attach the
+ * transaction to the checkpoint context so we carry the busy extents through
+ * to checkpoint completion, and then unlock all the items in the transaction.
+ *
+ * For more specific information about the order of operations in
+ * xfs_log_commit_cil() please refer to the comments in
+ * xfs_trans_commit_iclog().
+ */
+int
+xfs_log_commit_cil(
+	struct xfs_mount	*mp,
+	struct xfs_trans	*tp,
+	struct xfs_log_vec	*log_vector,
+	xfs_lsn_t		*commit_lsn,
+	int			flags)
+{
+	struct log		*log = mp->m_log;
+	int			log_flags = 0;
+
+	if (flags & XFS_TRANS_RELEASE_LOG_RES)
+		log_flags = XFS_LOG_REL_PERM_RESERV;
+
+	if (XLOG_FORCED_SHUTDOWN(log)) {
+		xlog_cil_free_logvec(log_vector);
+		return XFS_ERROR(EIO);
+	}
+
+	/* lock out background commit */
+	down_read(&log->l_cilp->xc_ctx_lock);
+	xlog_cil_format_items(log, log_vector, tp->t_ticket, commit_lsn);
+
+	/* check we didn't blow the reservation */
+	if (tp->t_ticket->t_curr_res < 0)
+		xlog_print_tic_res(log->l_mp, tp->t_ticket);
+
+	/* attach the transaction to the CIL if it has any busy extents */
+	if (!list_empty(&tp->t_busy)) {
+		spin_lock(&log->l_cilp->xc_cil_lock);
+		list_splice_init(&tp->t_busy,
+					&log->l_cilp->xc_ctx->busy_extents);
+		spin_unlock(&log->l_cilp->xc_cil_lock);
+	}
+
+	tp->t_commit_lsn = *commit_lsn;
+	xfs_log_done(mp, tp->t_ticket, NULL, log_flags);
+	xfs_trans_unreserve_and_mod_sb(tp);
+
+	/* background commit is allowed again */
+	up_read(&log->l_cilp->xc_ctx_lock);
+	return 0;
+}
+
+/*
+ * Mark all items committed and clear busy extents. We free the log vector
+ * chains in a separate pass so that we unpin the log items as quickly as
+ * possible.
+ */
+static void
+xlog_cil_committed(
+	void	*args,
+	int	abort)
+{
+	struct xfs_cil_ctx	*ctx = args;
+	struct xfs_log_vec	*lv;
+	int			abortflag = abort ? XFS_LI_ABORTED : 0;
+	struct xfs_busy_extent	*busyp, *n;
+
+	/* unpin all the log items */
+	for (lv = ctx->lv_chain; lv; lv = lv->lv_next ) {
+		xfs_trans_item_committed(lv->lv_item, ctx->start_lsn,
+							abortflag);
+	}
+
+	list_for_each_entry_safe(busyp, n, &ctx->busy_extents, list)
+		xfs_alloc_busy_clear(ctx->cil->xc_log->l_mp, busyp);
+
+	spin_lock(&ctx->cil->xc_cil_lock);
+	list_del(&ctx->committing);
+	spin_unlock(&ctx->cil->xc_cil_lock);
+
+	xlog_cil_free_logvec(ctx->lv_chain);
+	kmem_free(ctx);
+}
+
+/*
+ * Push the Committed Item List to the log. If the push_now flag is not set,
+ * then it is a background flush and so we can chose to ignore it.
+ */
+int
+xlog_cil_push(
+	struct log		*log,
+	int			push_now)
+{
+	struct xfs_cil		*cil = log->l_cilp;
+	struct xfs_log_vec	*lv;
+	struct xfs_cil_ctx	*ctx;
+	struct xfs_cil_ctx	*new_ctx;
+	struct xlog_in_core	*commit_iclog;
+	struct xlog_ticket	*tic;
+	int			num_lv;
+	int			num_iovecs;
+	int			len;
+	int			error = 0;
+	struct xfs_trans_header thdr;
+	struct xfs_log_iovec	lhdr;
+	struct xfs_log_vec	lvhdr = { NULL };
+	xfs_lsn_t		commit_lsn;
+
+	if (!cil)
+		return 0;
+
+	/* XXX: don't sleep for background? */
+	new_ctx = kmem_zalloc(sizeof(*new_ctx), KM_SLEEP|KM_NOFS);
+	new_ctx->ticket = xlog_cil_ticket_alloc(log);
+
+	/* lock out transaction commit */
+	down_write(&cil->xc_ctx_lock);
+	ctx = cil->xc_ctx;
+
+	/* check if we've anything to push */
+	if (list_empty(&cil->xc_cil))
+		goto out_skip;
+
+	/*
+	 * pull all the log vectors off the items in the CIL, and
+	 * remove the items from the CIL. We don't need the CIL lock
+	 * here because it's only needed on the transaction commit
+	 * side which is currently locked out by the flush lock.
+	 */
+	lv = NULL;
+	num_lv = 0;
+	num_iovecs = 0;
+	len = 0;
+	while (!list_empty(&cil->xc_cil)) {
+		struct xfs_log_item	*item;
+		int			i;
+
+		item = list_first_entry(&cil->xc_cil,
+					struct xfs_log_item, li_cil);
+		list_del_init(&item->li_cil);
+		if (!ctx->lv_chain)
+			ctx->lv_chain = item->li_lv;
+		else
+			lv->lv_next = item->li_lv;
+		lv = item->li_lv;
+		item->li_lv = NULL;
+
+		num_lv++;
+		num_iovecs += lv->lv_niovecs;
+		for (i = 0; i < lv->lv_niovecs; i++)
+			len += lv->lv_iovecp[i].i_len;
+	}
+
+	/*
+	 * initialise the new context and attach it to the CIL. Then attach
+	 * the current context to the CIL committing lsit so it can be found
+	 * during log forces to extract the commit lsn of the sequence that
+	 * needs to be forced.
+	 */
+	INIT_LIST_HEAD(&new_ctx->committing);
+	INIT_LIST_HEAD(&new_ctx->busy_extents);
+	new_ctx->sequence = ctx->sequence + 1;
+	new_ctx->cil = cil;
+	cil->xc_ctx = new_ctx;
+
+	/*
+	 * The switch is now done, so we can drop the context lock and move out
+	 * of a shared context. We can't just go straight to the commit record,
+	 * though - we need to synchronise with previous and future commits so
+	 * that the commit records are correctly ordered in the log to ensure
+	 * that we process items during log IO completion in the correct order.
+	 *
+	 * For example, if we get an EFI in one checkpoint and the EFD in the
+	 * next (e.g. due to log forces), we do not want the checkpoint with
+	 * the EFD to be committed before the checkpoint with the EFI.  Hence
+	 * we must strictly order the commit records of the checkpoints so
+	 * that: a) the checkpoint callbacks are attached to the iclogs in the
+	 * correct order; and b) the checkpoints are replayed in correct order
+	 * in log recovery.
+	 *
+	 * Hence we need to add this context to the committing context list so
+	 * that higher sequences will wait for us to write out a commit record
+	 * before they do.
+	 */
+	spin_lock(&cil->xc_cil_lock);
+	list_add(&ctx->committing, &cil->xc_committing);
+	spin_unlock(&cil->xc_cil_lock);
+	up_write(&cil->xc_ctx_lock);
+
+	/*
+	 * Build a checkpoint transaction header and write it to the log to
+	 * begin the transaction. We need to account for the space used by the
+	 * transaction header here as it is not accounted for in xlog_write().
+	 *
+	 * The LSN we need to pass to the log items on transaction commit is
+	 * the LSN reported by the first log vector write. If we use the commit
+	 * record lsn then we can move the tail beyond the grant write head.
+	 */
+	tic = ctx->ticket;
+	thdr.th_magic = XFS_TRANS_HEADER_MAGIC;
+	thdr.th_type = XFS_TRANS_CHECKPOINT;
+	thdr.th_tid = tic->t_tid;
+	thdr.th_num_items = num_iovecs;
+	lhdr.i_addr = (xfs_caddr_t)&thdr;
+	lhdr.i_len = sizeof(xfs_trans_header_t);
+	lhdr.i_type = XLOG_REG_TYPE_TRANSHDR;
+	tic->t_curr_res -= lhdr.i_len + sizeof(xlog_op_header_t);
+
+	lvhdr.lv_niovecs = 1;
+	lvhdr.lv_iovecp = &lhdr;
+	lvhdr.lv_next = ctx->lv_chain;
+
+	error = xlog_write(log, &lvhdr, tic, &ctx->start_lsn, NULL, 0);
+	if (error)
+		goto out_abort;
+
+	/*
+	 * now that we've written the checkpoint into the log, strictly
+	 * order the commit records so replay will get them in the right order.
+	 */
+restart:
+	spin_lock(&cil->xc_cil_lock);
+	list_for_each_entry(new_ctx, &cil->xc_committing, committing) {
+		/*
+		 * Higher sequences will wait for this one so skip them.
+		 * Don't wait for own own sequence, either.
+		 */
+		if (new_ctx->sequence >= ctx->sequence)
+			continue;
+		if (!new_ctx->commit_lsn) {
+			/*
+			 * It is still being pushed! Wait for the push to
+			 * complete, then start again from the beginning.
+			 */
+			sv_wait(&cil->xc_commit_wait, 0, &cil->xc_cil_lock, 0);
+			goto restart;
+		}
+	}
+	spin_unlock(&cil->xc_cil_lock);
+
+	commit_lsn = xfs_log_done(log->l_mp, tic, &commit_iclog, 0);
+	if (error || commit_lsn == -1)
+		goto out_abort;
+
+	/* attach all the transactions w/ busy extents to iclog */
+	ctx->log_cb.cb_func = xlog_cil_committed;
+	ctx->log_cb.cb_arg = ctx;
+	error = xfs_log_notify(log->l_mp, commit_iclog, &ctx->log_cb);
+	if (error)
+		goto out_abort;
+
+	/*
+	 * now the checkpoint commit is complete and we've attached the
+	 * callbacks to the iclog we can assign the commit LSN to the context
+	 * and wake up anyone who is waiting for the commit to complete.
+	 */
+	spin_lock(&cil->xc_cil_lock);
+	ctx->commit_lsn = commit_lsn;
+	sv_broadcast(&cil->xc_commit_wait);
+	spin_unlock(&cil->xc_cil_lock);
+
+	/* release the hounds! */
+	return xfs_log_release_iclog(log->l_mp, commit_iclog);
+
+out_skip:
+	up_write(&cil->xc_ctx_lock);
+	xfs_log_ticket_put(new_ctx->ticket);
+	kmem_free(new_ctx);
+	return 0;
+
+out_abort:
+	xlog_cil_committed(ctx, XFS_LI_ABORTED);
+	return XFS_ERROR(EIO);
+}
+
+/*
+ * Conditionally push the CIL based on the sequence passed in.
+ *
+ * We only need to push if we haven't already pushed the sequence
+ * number given. Hence the only time we will trigger a push here is
+ * if the push sequence is the same as the current context.
+ *
+ * We return the current commit lsn to allow the callers to determine if a
+ * iclog flush is necessary following this call.
+ *
+ * XXX: Initially, just push the CIL unconditionally and return whatever
+ * commit lsn is there. It'll be empty, so this is broken for now.
+ */
+xfs_lsn_t
+xlog_cil_push_lsn(
+	struct log	*log,
+	xfs_lsn_t	push_seq)
+{
+	struct xfs_cil		*cil = log->l_cilp;
+	struct xfs_cil_ctx	*ctx;
+	xfs_lsn_t		commit_lsn = NULLCOMMITLSN;
+
+restart:
+	down_write(&cil->xc_ctx_lock);
+	ASSERT(push_seq <= cil->xc_ctx->sequence);
+
+	/* check to see if we need to force out the current context */
+	if (push_seq == cil->xc_ctx->sequence) {
+		up_write(&cil->xc_ctx_lock);
+		xlog_cil_push(log, 1);
+		goto restart;
+	}
+
+	/*
+	 * See if we can find a previous sequence still committing.
+	 * We can drop the flush lock as soon as we have the cil lock
+	 * because we are now only comparing contexts protected by
+	 * the cil lock.
+	 *
+	 * We need to wait for all previous sequence commits to complete
+	 * before allowing the force of push_seq to go ahead. Hence block
+	 * on commits for those as well.
+	 */
+	spin_lock(&cil->xc_cil_lock);
+	up_write(&cil->xc_ctx_lock);
+	list_for_each_entry(ctx, &cil->xc_committing, committing) {
+		if (ctx->sequence > push_seq)
+			continue;
+		if (!ctx->commit_lsn) {
+			/*
+			 * It is still being pushed! Wait for the push to
+			 * complete, then start again from the beginning.
+			 */
+			sv_wait(&cil->xc_commit_wait, 0, &cil->xc_cil_lock, 0);
+			goto restart;
+		}
+		if (ctx->sequence != push_seq)
+			continue;
+		/* found it! */
+		commit_lsn = ctx->commit_lsn;
+	}
+	spin_unlock(&cil->xc_cil_lock);
+	return commit_lsn;
+}