xfs: move CIL ordering to the logvec chain

Adding a list_sort() call to the CIL push work while the xc_ctx_lock
is held exclusively has resulted in fairly long lock hold times and
that stops all front end transaction commits from making progress.

We can move the sorting out of the xc_ctx_lock if we can transfer
the ordering information to the log vectors as they are detached
from the log items and then we can sort the log vectors.  With these
changes, we can move the list_sort() call to just before we call
xlog_write() when we aren't holding any locks at all.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Author: dgchinner

fs/xfs/xfs_log.h

@@ -10,6 +10,7 @@ struct xfs_cil_ctx;
 
 struct xfs_log_vec {
 	struct list_head	lv_list;	/* CIL lv chain ptrs */
+	uint32_t		lv_order_id;	/* chain ordering info */
 	int			lv_niovecs;	/* number of iovecs in lv */
 	struct xfs_log_iovec	*lv_iovecp;	/* iovec array */
 	struct xfs_log_item	*lv_item;	/* owner */

fs/xfs/xfs_log_cil.c

@@ -1095,10 +1095,10 @@ xlog_cil_order_cmp(
 	const struct list_head	*a,
 	const struct list_head	*b)
 {
-	struct xfs_log_item	*l1 = container_of(a, struct xfs_log_item, li_cil);
-	struct xfs_log_item	*l2 = container_of(b, struct xfs_log_item, li_cil);
+	struct xfs_log_vec	*l1 = container_of(a, struct xfs_log_vec, lv_list);
+	struct xfs_log_vec	*l2 = container_of(b, struct xfs_log_vec, lv_list);
 
-	return l1->li_order_id > l2->li_order_id;
+	return l1->lv_order_id > l2->lv_order_id;
 }
 
 /*
@@ -1117,8 +1117,6 @@ xlog_cil_build_lv_chain(
 	uint32_t		*num_iovecs,
 	uint32_t		*num_bytes)
 {
-	list_sort(NULL, &ctx->log_items, xlog_cil_order_cmp);
-
 	while (!list_empty(&ctx->log_items)) {
 		struct xfs_log_item	*item;
 		struct xfs_log_vec	*lv;
@@ -1133,6 +1131,7 @@ xlog_cil_build_lv_chain(
 		}
 
 		lv = item->li_lv;
+		lv->lv_order_id = item->li_order_id;
 
 		/* we don't write ordered log vectors */
 		if (lv->lv_buf_len != XFS_LOG_VEC_ORDERED)
@@ -1292,6 +1291,13 @@ xlog_cil_push_work(
 	spin_unlock(&cil->xc_push_lock);
 	up_write(&cil->xc_ctx_lock);
 
+	/*
+	 * Sort the log vector chain before we add the transaction headers.
+	 * This ensures we always have the transaction headers at the start
+	 * of the chain.
+	 */
+	list_sort(NULL, &ctx->lv_chain, xlog_cil_order_cmp);
+
 	/*
 	 * 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