[XFS] Use a cursor for AIL traversal.

To replace the current generation number ensuring sanity of the AIL
traversal, replace it with an external cursor that is linked to the AIL.

Basically, we store the next item in the cursor whenever we want to drop
the AIL lock to do something to the current item. When we regain the lock.
the current item may already be free, so we can't reference it, but the
next item in the traversal is already held in the cursor.

When we move or delete an object, we search all the active cursors and if
there is an item match we clear the cursor(s) that point to the object.
This forces the traversal to restart transparently.

We don't invalidate the cursor on insert because the cursor still points
to a valid item. If the intem is inserted between the current item and the
cursor it does not matter; the traversal is considered to be past the
insertion point so it will be picked up in the next traversal.

Hence traversal restarts pretty much disappear altogether with this method
of traversal, which should substantially reduce the overhead of pushing on
a busy AIL.

Version 2 o add restart logic o comment cursor interface o minor cleanups

SGI-PV: 988143

SGI-Modid: xfs-linux-melb:xfs-kern:32347a
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>

fs/xfs/xfs_log.c

@@ -900,7 +900,7 @@ xfs_log_move_tail(xfs_mount_t	*mp,
 int
 xfs_log_need_covered(xfs_mount_t *mp)
 {
-	int		needed = 0, gen;
+	int		needed = 0;
 	xlog_t		*log = mp->m_log;
 
 	if (!xfs_fs_writable(mp))
@@ -909,7 +909,7 @@ xfs_log_need_covered(xfs_mount_t *mp)
 	spin_lock(&log->l_icloglock);
 	if (((log->l_covered_state == XLOG_STATE_COVER_NEED) ||
 		(log->l_covered_state == XLOG_STATE_COVER_NEED2))
-			&& !xfs_trans_first_ail(mp, &gen)
+			&& !xfs_trans_first_ail(mp, NULL)
 			&& xlog_iclogs_empty(log)) {
 		if (log->l_covered_state == XLOG_STATE_COVER_NEED)
 			log->l_covered_state = XLOG_STATE_COVER_DONE;

fs/xfs/xfs_log_recover.c

@@ -54,10 +54,8 @@ STATIC void	xlog_recover_insert_item_backq(xlog_recover_item_t **q,
 					       xlog_recover_item_t *item);
 #if defined(DEBUG)
 STATIC void	xlog_recover_check_summary(xlog_t *);
-STATIC void	xlog_recover_check_ail(xfs_mount_t *, xfs_log_item_t *, int);
 #else
 #define	xlog_recover_check_summary(log)
-#define	xlog_recover_check_ail(mp, lip, gen)
 #endif
 
 
@@ -2710,8 +2708,8 @@ xlog_recover_do_efd_trans(
 	xfs_efd_log_format_t	*efd_formatp;
 	xfs_efi_log_item_t	*efip = NULL;
 	xfs_log_item_t		*lip;
-	int			gen;
 	__uint64_t		efi_id;
+	struct xfs_ail_cursor	cur;
 
 	if (pass == XLOG_RECOVER_PASS1) {
 		return;
@@ -2730,7 +2728,8 @@ xlog_recover_do_efd_trans(
 	 */
 	mp = log->l_mp;
 	spin_lock(&mp->m_ail_lock);
-	lip = xfs_trans_first_ail(mp, &gen);
+	xfs_trans_ail_cursor_init(mp->m_ail, &cur);
+	lip = xfs_trans_first_ail(mp, &cur);
 	while (lip != NULL) {
 		if (lip->li_type == XFS_LI_EFI) {
 			efip = (xfs_efi_log_item_t *)lip;
@@ -2741,11 +2740,13 @@ xlog_recover_do_efd_trans(
 				 */
 				xfs_trans_delete_ail(mp, lip);
 				xfs_efi_item_free(efip);
-				return;
+				spin_lock(&mp->m_ail_lock);
+				break;
 			}
 		}
-		lip = xfs_trans_next_ail(mp, lip, &gen, NULL);
+		lip = xfs_trans_next_ail(mp, &cur);
 	}
+	xfs_trans_ail_cursor_done(mp->m_ail, &cur);
 	spin_unlock(&mp->m_ail_lock);
 }
 
@@ -3029,33 +3030,6 @@ xlog_recover_process_efi(
 	return error;
 }
 
-/*
- * Verify that once we've encountered something other than an EFI
- * in the AIL that there are no more EFIs in the AIL.
- */
-#if defined(DEBUG)
-STATIC void
-xlog_recover_check_ail(
-	xfs_mount_t		*mp,
-	xfs_log_item_t		*lip,
-	int			gen)
-{
-	int			orig_gen = gen;
-
-	do {
-		ASSERT(lip->li_type != XFS_LI_EFI);
-		lip = xfs_trans_next_ail(mp, lip, &gen, NULL);
-		/*
-		 * The check will be bogus if we restart from the
-		 * beginning of the AIL, so ASSERT that we don't.
-		 * We never should since we're holding the AIL lock
-		 * the entire time.
-		 */
-		ASSERT(gen == orig_gen);
-	} while (lip != NULL);
-}
-#endif	/* DEBUG */
-
 /*
  * When this is called, all of the EFIs which did not have
  * corresponding EFDs should be in the AIL.  What we do now
@@ -3080,20 +3054,25 @@ xlog_recover_process_efis(
 {
 	xfs_log_item_t		*lip;
 	xfs_efi_log_item_t	*efip;
-	int			gen;
 	xfs_mount_t		*mp;
 	int			error = 0;
+	struct xfs_ail_cursor	cur;
 
 	mp = log->l_mp;
 	spin_lock(&mp->m_ail_lock);
 
-	lip = xfs_trans_first_ail(mp, &gen);
+	xfs_trans_ail_cursor_init(mp->m_ail, &cur);
+	lip = xfs_trans_first_ail(mp, &cur);
 	while (lip != NULL) {
 		/*
 		 * We're done when we see something other than an EFI.
+		 * There should be no EFIs left in the AIL now.
 		 */
 		if (lip->li_type != XFS_LI_EFI) {
-			xlog_recover_check_ail(mp, lip, gen);
+#ifdef DEBUG
+			for (; lip; lip = xfs_trans_next_ail(mp, &cur))
+				ASSERT(lip->li_type != XFS_LI_EFI);
+#endif
 			break;
 		}
 
@@ -3102,17 +3081,19 @@ xlog_recover_process_efis(
 		 */
 		efip = (xfs_efi_log_item_t *)lip;
 		if (efip->efi_flags & XFS_EFI_RECOVERED) {
-			lip = xfs_trans_next_ail(mp, lip, &gen, NULL);
+			lip = xfs_trans_next_ail(mp, &cur);
 			continue;
 		}
 
 		spin_unlock(&mp->m_ail_lock);
 		error = xlog_recover_process_efi(mp, efip);
-		if (error)
-			return error;
 		spin_lock(&mp->m_ail_lock);
-		lip = xfs_trans_next_ail(mp, lip, &gen, NULL);
+		if (error)
+			goto out;
+		lip = xfs_trans_next_ail(mp, &cur);
 	}
+out:
+	xfs_trans_ail_cursor_done(mp->m_ail, &cur);
 	spin_unlock(&mp->m_ail_lock);
 	return error;
 }

fs/xfs/xfs_trans_ail.c

@@ -98,6 +98,115 @@ xfs_trans_push_ail(
 	}
 }
 
+/*
+ * AIL traversal cursor initialisation.
+ *
+ * The cursor keeps track of where our current traversal is up
+ * to by tracking the next ƣtem in the list for us. However, for
+ * this to be safe, removing an object from the AIL needs to invalidate
+ * any cursor that points to it. hence the traversal cursor needs to
+ * be linked to the struct xfs_ail so that deletion can search all the
+ * active cursors for invalidation.
+ *
+ * We don't link the push cursor because it is embedded in the struct
+ * xfs_ail and hence easily findable.
+ */
+void
+xfs_trans_ail_cursor_init(
+	struct xfs_ail		*ailp,
+	struct xfs_ail_cursor	*cur)
+{
+	cur->item = NULL;
+	if (cur == &ailp->xa_cursors)
+		return;
+
+	cur->next = ailp->xa_cursors.next;
+	ailp->xa_cursors.next = cur;
+}
+
+/*
+ * Set the cursor to the next item, because when we look
+ * up the cursor the current item may have been freed.
+ */
+STATIC void
+xfs_trans_ail_cursor_set(
+	struct xfs_ail		*ailp,
+	struct xfs_ail_cursor	*cur,
+	struct xfs_log_item	*lip)
+{
+	if (lip)
+		cur->item = xfs_ail_next(ailp, lip);
+}
+
+/*
+ * Get the next item in the traversal and advance the cursor.
+ * If the cursor was invalidated (inidicated by a lip of 1),
+ * restart the traversal.
+ */
+STATIC struct xfs_log_item *
+xfs_trans_ail_cursor_next(
+	struct xfs_ail		*ailp,
+	struct xfs_ail_cursor	*cur)
+{
+	struct xfs_log_item	*lip = cur->item;
+
+	if ((__psint_t)lip & 1)
+		lip = xfs_ail_min(ailp);
+	xfs_trans_ail_cursor_set(ailp, cur, lip);
+	return lip;
+}
+
+/*
+ * Invalidate any cursor that is pointing to this item. This is
+ * called when an item is removed from the AIL. Any cursor pointing
+ * to this object is now invalid and the traversal needs to be
+ * terminated so it doesn't reference a freed object. We set the
+ * cursor item to a value of 1 so we can distinguish between an
+ * invalidation and the end of the list when getting the next item
+ * from the cursor.
+ */
+STATIC void
+xfs_trans_ail_cursor_clear(
+	struct xfs_ail		*ailp,
+	struct xfs_log_item	*lip)
+{
+	struct xfs_ail_cursor	*cur;
+
+	/* need to search all cursors */
+	for (cur = &ailp->xa_cursors; cur; cur = cur->next) {
+		if (cur->item == lip)
+			cur->item = (struct xfs_log_item *)
+					((__psint_t)cur->item | 1);
+	}
+}
+
+/*
+ * Now that the traversal is complete, we need to remove the cursor
+ * from the list of traversing cursors. Avoid removing the embedded
+ * push cursor, but use the fact it is alway present to make the
+ * list deletion simple.
+ */
+void
+xfs_trans_ail_cursor_done(
+	struct xfs_ail		*ailp,
+	struct xfs_ail_cursor	*done)
+{
+	struct xfs_ail_cursor	*prev = NULL;
+	struct xfs_ail_cursor	*cur;
+
+	done->item = NULL;
+	if (done == &ailp->xa_cursors)
+		return;
+	prev = &ailp->xa_cursors;
+	for (cur = prev->next; cur; prev = cur, cur = prev->next) {
+		if (cur == done) {
+			prev->next = cur->next;
+			break;
+		}
+	}
+	ASSERT(cur);
+}
+
 /*
  * Return the item in the AIL with the current lsn.
  * Return the current tree generation number for use
@@ -105,20 +214,22 @@ xfs_trans_push_ail(
  */
 STATIC xfs_log_item_t *
 xfs_trans_first_push_ail(
-	xfs_mount_t	*mp,
-	int		*gen,
-	xfs_lsn_t	lsn)
+	struct xfs_ail		*ailp,
+	struct xfs_ail_cursor	*cur,
+	xfs_lsn_t		lsn)
 {
-	xfs_log_item_t	*lip;
+	xfs_log_item_t		*lip;
 
-	lip = xfs_ail_min(mp->m_ail);
-	*gen = (int)mp->m_ail->xa_gen;
+	lip = xfs_ail_min(ailp);
+	xfs_trans_ail_cursor_set(ailp, cur, lip);
 	if (lsn == 0)
 		return lip;
 
-	list_for_each_entry(lip, &mp->m_ail->xa_ail, li_ail) {
-		if (XFS_LSN_CMP(lip->li_lsn, lsn) >= 0)
+	list_for_each_entry(lip, &ailp->xa_ail, li_ail) {
+		if (XFS_LSN_CMP(lip->li_lsn, lsn) >= 0) {
+			xfs_trans_ail_cursor_set(ailp, cur, lip);
 			return lip;
+		}
 	}
 
 	return NULL;
@@ -137,22 +248,21 @@ xfsaild_push(
 	xfs_lsn_t	target =  ailp->xa_target;
 	xfs_lsn_t	lsn;
 	xfs_log_item_t	*lip;
-	int		gen;
-	int		restarts;
 	int		flush_log, count, stuck;
 	xfs_mount_t	*mp = ailp->xa_mount;
-
-#define	XFS_TRANS_PUSH_AIL_RESTARTS	10
+	struct xfs_ail_cursor	*cur = &ailp->xa_cursors;
 
 	spin_lock(&mp->m_ail_lock);
-	lip = xfs_trans_first_push_ail(mp, &gen, *last_lsn);
+	xfs_trans_ail_cursor_init(ailp, cur);
+	lip = xfs_trans_first_push_ail(ailp, cur, *last_lsn);
 	if (!lip || XFS_FORCED_SHUTDOWN(mp)) {
 		/*
 		 * AIL is empty or our push has reached the end.
 		 */
+		xfs_trans_ail_cursor_done(ailp, cur);
 		spin_unlock(&mp->m_ail_lock);
 		last_pushed_lsn = 0;
-		goto out;
+		return tout;
 	}
 
 	XFS_STATS_INC(xs_push_ail);
@@ -170,7 +280,7 @@ xfsaild_push(
 	 */
 	tout = 10;
 	lsn = lip->li_lsn;
-	flush_log = stuck = count = restarts = 0;
+	flush_log = stuck = count = 0;
 	while ((XFS_LSN_CMP(lip->li_lsn, target) < 0)) {
 		int	lock_result;
 		/*
@@ -245,13 +355,12 @@ xfsaild_push(
 		if (stuck > 100)
 			break;
 
-		lip = xfs_trans_next_ail(mp, lip, &gen, &restarts);
+		lip = xfs_trans_ail_cursor_next(ailp, cur);
 		if (lip == NULL)
 			break;
-		if (restarts > XFS_TRANS_PUSH_AIL_RESTARTS)
-			break;
 		lsn = lip->li_lsn;
 	}
+	xfs_trans_ail_cursor_done(ailp, cur);
 	spin_unlock(&mp->m_ail_lock);
 
 	if (flush_log) {
@@ -275,8 +384,7 @@ xfsaild_push(
 		 */
 		tout += 20;
 		last_pushed_lsn = 0;
-	} else if ((restarts > XFS_TRANS_PUSH_AIL_RESTARTS) ||
-		   ((stuck * 100) / count > 90)) {
+	} else if ((stuck * 100) / count > 90) {
 		/*
 		 * Either there is a lot of contention on the AIL or we
 		 * are stuck due to operations in progress. "Stuck" in this
@@ -288,7 +396,6 @@ xfsaild_push(
 		 */
 		tout += 10;
 	}
-out:
 	*last_lsn = last_pushed_lsn;
 	return tout;
 }	/* xfsaild_push */
@@ -348,9 +455,6 @@ xfs_trans_unlocked_item(
  * we move in the AIL is the minimum one, update the tail lsn in the
  * log manager.
  *
- * Increment the AIL's generation count to indicate that the tree
- * has changed.
- *
  * This function must be called with the AIL lock held.  The lock
  * is dropped before returning.
  */
@@ -368,14 +472,13 @@ xfs_trans_update_ail(
 	if (lip->li_flags & XFS_LI_IN_AIL) {
 		dlip = xfs_ail_delete(mp->m_ail, lip);
 		ASSERT(dlip == lip);
+		xfs_trans_ail_cursor_clear(mp->m_ail, dlip);
 	} else {
 		lip->li_flags |= XFS_LI_IN_AIL;
 	}
 
 	lip->li_lsn = lsn;
-
 	xfs_ail_insert(mp->m_ail, lip);
-	mp->m_ail->xa_gen++;
 
 	if (mlip == dlip) {
 		mlip = xfs_ail_min(mp->m_ail);
@@ -415,11 +518,11 @@ xfs_trans_delete_ail(
 		mlip = xfs_ail_min(mp->m_ail);
 		dlip = xfs_ail_delete(mp->m_ail, lip);
 		ASSERT(dlip == lip);
+		xfs_trans_ail_cursor_clear(mp->m_ail, dlip);
 
 
 		lip->li_flags &= ~XFS_LI_IN_AIL;
 		lip->li_lsn = 0;
-		mp->m_ail->xa_gen++;
 
 		if (mlip == dlip) {
 			mlip = xfs_ail_min(mp->m_ail);
@@ -455,46 +558,29 @@ xfs_trans_delete_ail(
  */
 xfs_log_item_t *
 xfs_trans_first_ail(
-	xfs_mount_t	*mp,
-	int		*gen)
+	struct xfs_mount	*mp,
+	struct xfs_ail_cursor	*cur)
 {
-	xfs_log_item_t	*lip;
+	xfs_log_item_t		*lip;
+	struct xfs_ail		*ailp = mp->m_ail;
 
-	lip = xfs_ail_min(mp->m_ail);
-	*gen = (int)mp->m_ail->xa_gen;
+	lip = xfs_ail_min(ailp);
+	xfs_trans_ail_cursor_set(ailp, cur, lip);
 
 	return lip;
 }
 
 /*
- * If the generation count of the tree has not changed since the
- * caller last took something from the AIL, then return the elmt
- * in the tree which follows the one given.  If the count has changed,
- * then return the minimum elmt of the AIL and bump the restarts counter
- * if one is given.
+ * Grab the next item in the AIL from the cursor passed in.
  */
 xfs_log_item_t *
 xfs_trans_next_ail(
-	xfs_mount_t	*mp,
-	xfs_log_item_t	*lip,
-	int		*gen,
-	int		*restarts)
+	struct xfs_mount	*mp,
+	struct xfs_ail_cursor	*cur)
 {
-	xfs_log_item_t	*nlip;
-
-	ASSERT(mp && lip && gen);
-	if (mp->m_ail->xa_gen == *gen) {
-		nlip = xfs_ail_next(mp->m_ail, lip);
-	} else {
-		nlip = xfs_ail_min(mp->m_ail);
-		*gen = (int)mp->m_ail->xa_gen;
-		if (restarts != NULL) {
-			XFS_STATS_INC(xs_push_ail_restarts);
-			(*restarts)++;
-		}
-	}
+	struct xfs_ail		*ailp = mp->m_ail;
 
-	return (nlip);
+	return xfs_trans_ail_cursor_next(ailp, cur);
 }
 
 
@@ -517,6 +603,7 @@ xfs_trans_ail_init(
 	xfs_mount_t	*mp)
 {
 	struct xfs_ail	*ailp;
+	int		error;
 
 	ailp = kmem_zalloc(sizeof(struct xfs_ail), KM_MAYFAIL);
 	if (!ailp)
@@ -524,7 +611,15 @@ xfs_trans_ail_init(
 
 	ailp->xa_mount = mp;
 	INIT_LIST_HEAD(&ailp->xa_ail);
-	return xfsaild_start(ailp);
+	error = xfsaild_start(ailp);
+	if (error)
+		goto out_free_ailp;
+	mp->m_ail = ailp;
+	return 0;
+
+out_free_ailp:
+	kmem_free(ailp);
+	return error;
 }
 
 void

fs/xfs/xfs_trans_priv.h

@@ -44,20 +44,33 @@ xfs_log_busy_slot_t		*xfs_trans_add_busy(xfs_trans_t *tp,
 						    xfs_extlen_t idx);
 
 /*
- * From xfs_trans_ail.c
+ * AIL traversal cursor.
+ *
+ * Rather than using a generation number for detecting changes in the ail, use
+ * a cursor that is protected by the ail lock. The aild cursor exists in the
+ * struct xfs_ail, but other traversals can declare it on the stack and link it
+ * to the ail list.
+ *
+ * When an object is deleted from or moved int the AIL, the cursor list is
+ * searched to see if the object is a designated cursor item. If it is, it is
+ * deleted from the cursor so that the next time the cursor is used traversal
+ * will return to the start.
+ *
+ * This means a traversal colliding with a removal will cause a restart of the
+ * list scan, rather than any insertion or deletion anywhere in the list. The
+ * low bit of the item pointer is set if the cursor has been invalidated so
+ * that we can tell the difference between invalidation and reaching the end
+ * of the list to trigger traversal restarts.
  */
-void			xfs_trans_update_ail(struct xfs_mount *mp,
-				     struct xfs_log_item *lip, xfs_lsn_t lsn)
-				     __releases(mp->m_ail_lock);
-void			xfs_trans_delete_ail(struct xfs_mount *mp,
-				     struct xfs_log_item *lip)
-				     __releases(mp->m_ail_lock);
-struct xfs_log_item	*xfs_trans_first_ail(struct xfs_mount *, int *);
-struct xfs_log_item	*xfs_trans_next_ail(struct xfs_mount *,
-				     struct xfs_log_item *, int *, int *);
+struct xfs_ail_cursor {
+	struct xfs_ail_cursor	*next;
+	struct xfs_log_item	*item;
+};
 
 /*
- * AIL push thread support
+ * Private AIL structures.
+ *
+ * Eventually we need to drive the locking in here as well.
  */
 struct xfs_ail {
 	struct xfs_mount	*xa_mount;
@@ -65,8 +78,28 @@ struct xfs_ail {
 	uint			xa_gen;
 	struct task_struct	*xa_task;
 	xfs_lsn_t		xa_target;
+	struct xfs_ail_cursor	xa_cursors;
 };
 
+/*
+ * From xfs_trans_ail.c
+ */
+void			xfs_trans_update_ail(struct xfs_mount *mp,
+				     struct xfs_log_item *lip, xfs_lsn_t lsn)
+				     __releases(mp->m_ail_lock);
+void			xfs_trans_delete_ail(struct xfs_mount *mp,
+				     struct xfs_log_item *lip)
+				     __releases(mp->m_ail_lock);
+struct xfs_log_item	*xfs_trans_first_ail(struct xfs_mount *mp,
+					struct xfs_ail_cursor *cur);
+struct xfs_log_item	*xfs_trans_next_ail(struct xfs_mount *mp,
+					struct xfs_ail_cursor *cur);
+
+void xfs_trans_ail_cursor_init(struct xfs_ail *ailp,
+					struct xfs_ail_cursor *cur);
+void xfs_trans_ail_cursor_done(struct xfs_ail *ailp,
+					struct xfs_ail_cursor *cur);
+
 long	xfsaild_push(struct xfs_ail *, xfs_lsn_t *);
 void	xfsaild_wakeup(struct xfs_ail *, xfs_lsn_t);
 int	xfsaild_start(struct xfs_ail *);