xfs: allow assigning the tail lsn with the AIL lock held

Provide a variant of xlog_assign_tail_lsn that has the AIL lock already
held.  By doing so we do an additional atomic_read + atomic_set under
the lock, which comes down to two instructions.

Switch xfs_trans_ail_update_bulk and xfs_trans_ail_delete_bulk to the
new version to reduce the number of lock roundtrips, and prepare for
a new addition that would require a third lock roundtrip in
xfs_trans_ail_delete_bulk.  This addition is also the reason for
slightly rearranging the conditionals and relying on xfs_log_space_wake
for checking that the filesystem has been shut down internally.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_log.c

@@ -916,27 +916,42 @@ xfs_log_need_covered(xfs_mount_t *mp)
  * We may be holding the log iclog lock upon entering this routine.
  */
 xfs_lsn_t
-xlog_assign_tail_lsn(
+xlog_assign_tail_lsn_locked(
 	struct xfs_mount	*mp)
 {
-	xfs_lsn_t		tail_lsn;
 	struct log		*log = mp->m_log;
+	struct xfs_log_item	*lip;
+	xfs_lsn_t		tail_lsn;
+
+	assert_spin_locked(&mp->m_ail->xa_lock);
 
 	/*
 	 * To make sure we always have a valid LSN for the log tail we keep
 	 * track of the last LSN which was committed in log->l_last_sync_lsn,
-	 * and use that when the AIL was empty and xfs_ail_min_lsn returns 0.
-	 *
-	 * If the AIL has been emptied we also need to wake any process
-	 * waiting for this condition.
+	 * and use that when the AIL was empty.
 	 */
-	tail_lsn = xfs_ail_min_lsn(mp->m_ail);
-	if (!tail_lsn)
+	lip = xfs_ail_min(mp->m_ail);
+	if (lip)
+		tail_lsn = lip->li_lsn;
+	else
 		tail_lsn = atomic64_read(&log->l_last_sync_lsn);
 	atomic64_set(&log->l_tail_lsn, tail_lsn);
 	return tail_lsn;
 }
 
+xfs_lsn_t
+xlog_assign_tail_lsn(
+	struct xfs_mount	*mp)
+{
+	xfs_lsn_t		tail_lsn;
+
+	spin_lock(&mp->m_ail->xa_lock);
+	tail_lsn = xlog_assign_tail_lsn_locked(mp);
+	spin_unlock(&mp->m_ail->xa_lock);
+
+	return tail_lsn;
+}
+
 /*
  * Return the space in the log between the tail and the head.  The head
  * is passed in the cycle/bytes formal parms.  In the special case where

fs/xfs/xfs_log.h

@@ -152,6 +152,7 @@ int	  xfs_log_mount(struct xfs_mount	*mp,
 			int		 	num_bblocks);
 int	  xfs_log_mount_finish(struct xfs_mount *mp);
 xfs_lsn_t xlog_assign_tail_lsn(struct xfs_mount *mp);
+xfs_lsn_t xlog_assign_tail_lsn_locked(struct xfs_mount *mp);
 void	  xfs_log_space_wake(struct xfs_mount *mp);
 int	  xfs_log_notify(struct xfs_mount	*mp,
 			 struct xlog_in_core	*iclog,

fs/xfs/xfs_trans_ail.c

@@ -79,7 +79,7 @@ xfs_ail_check(
  * Return a pointer to the first item in the AIL.  If the AIL is empty, then
  * return NULL.
  */
-static xfs_log_item_t *
+xfs_log_item_t *
 xfs_ail_min(
 	struct xfs_ail  *ailp)
 {
@@ -667,11 +667,15 @@ xfs_trans_ail_update_bulk(
 
 	if (!list_empty(&tmp))
 		xfs_ail_splice(ailp, cur, &tmp, lsn);
-	spin_unlock(&ailp->xa_lock);
 
-	if (mlip_changed && !XFS_FORCED_SHUTDOWN(ailp->xa_mount)) {
-		xlog_assign_tail_lsn(ailp->xa_mount);
+	if (mlip_changed) {
+		if (!XFS_FORCED_SHUTDOWN(ailp->xa_mount))
+			xlog_assign_tail_lsn_locked(ailp->xa_mount);
+		spin_unlock(&ailp->xa_lock);
+
 		xfs_log_space_wake(ailp->xa_mount);
+	} else {
+		spin_unlock(&ailp->xa_lock);
 	}
 }
 
@@ -729,11 +733,15 @@ xfs_trans_ail_delete_bulk(
 		if (mlip == lip)
 			mlip_changed = 1;
 	}
-	spin_unlock(&ailp->xa_lock);
 
-	if (mlip_changed && !XFS_FORCED_SHUTDOWN(ailp->xa_mount)) {
-		xlog_assign_tail_lsn(ailp->xa_mount);
+	if (mlip_changed) {
+		if (!XFS_FORCED_SHUTDOWN(ailp->xa_mount))
+			xlog_assign_tail_lsn_locked(ailp->xa_mount);
+		spin_unlock(&ailp->xa_lock);
+
 		xfs_log_space_wake(ailp->xa_mount);
+	} else {
+		spin_unlock(&ailp->xa_lock);
 	}
 }
 

fs/xfs/xfs_trans_priv.h

@@ -102,6 +102,7 @@ xfs_trans_ail_delete(
 
 void			xfs_ail_push(struct xfs_ail *, xfs_lsn_t);
 void			xfs_ail_push_all(struct xfs_ail *);
+struct xfs_log_item	*xfs_ail_min(struct xfs_ail  *ailp);
 xfs_lsn_t		xfs_ail_min_lsn(struct xfs_ail *ailp);
 
 struct xfs_log_item *	xfs_trans_ail_cursor_first(struct xfs_ail *ailp,