xfs: use WRITE_ONCE to update if_seq

This adds ordering of the updates and makes sure we always see the if_seq
update before the extent tree is modified.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_iext_tree.c

@@ -14,6 +14,7 @@
 #include "xfs_inode_fork.h"
 #include "xfs_trans_resv.h"
 #include "xfs_mount.h"
+#include "xfs_bmap.h"
 #include "xfs_trace.h"
 
 /*
@@ -612,6 +613,19 @@ xfs_iext_realloc_root(
 	cur->leaf = new;
 }
 
+/*
+ * Increment the sequence counter if we are on a COW fork.  This allows
+ * the writeback code to skip looking for a COW extent if the COW fork
+ * hasn't changed.  We use WRITE_ONCE here to ensure the update to the
+ * sequence counter is seen before the modifications to the extent
+ * tree itself take effect.
+ */
+static inline void xfs_iext_inc_seq(struct xfs_ifork *ifp, int state)
+{
+	if (state & BMAP_COWFORK)
+		WRITE_ONCE(ifp->if_seq, READ_ONCE(ifp->if_seq) + 1);
+}
+
 void
 xfs_iext_insert(
 	struct xfs_inode	*ip,
@@ -624,7 +638,7 @@ xfs_iext_insert(
 	struct xfs_iext_leaf	*new = NULL;
 	int			nr_entries, i;
 
-	ifp->if_seq++;
+	xfs_iext_inc_seq(ifp, state);
 
 	if (ifp->if_height == 0)
 		xfs_iext_alloc_root(ifp, cur);
@@ -866,7 +880,7 @@ xfs_iext_remove(
 	ASSERT(ifp->if_u1.if_root != NULL);
 	ASSERT(xfs_iext_valid(ifp, cur));
 
-	ifp->if_seq++;
+	xfs_iext_inc_seq(ifp, state);
 
 	nr_entries = xfs_iext_leaf_nr_entries(ifp, leaf, cur->pos) - 1;
 	for (i = cur->pos; i < nr_entries; i++)
@@ -974,7 +988,7 @@ xfs_iext_update_extent(
 {
 	struct xfs_ifork	*ifp = xfs_iext_state_to_fork(ip, state);
 
-	ifp->if_seq++;
+	xfs_iext_inc_seq(ifp, state);
 
 	if (cur->pos == 0) {
 		struct xfs_bmbt_irec	old;

fs/xfs/xfs_aops.c

@@ -351,7 +351,7 @@ xfs_map_blocks(
 	if (imap_valid &&
 	    (!xfs_inode_has_cow_data(ip) ||
 	     wpc->io_type == XFS_IO_COW ||
-	     wpc->cow_seq == ip->i_cowfp->if_seq))
+	     wpc->cow_seq == READ_ONCE(ip->i_cowfp->if_seq)))
 		return 0;
 
 	if (XFS_FORCED_SHUTDOWN(mp))
@@ -380,7 +380,7 @@ xfs_map_blocks(
 	    xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &imap))
 		cow_fsb = imap.br_startoff;
 	if (cow_fsb != NULLFILEOFF && cow_fsb <= offset_fsb) {
-		wpc->cow_seq = ip->i_cowfp->if_seq;
+		wpc->cow_seq = READ_ONCE(ip->i_cowfp->if_seq);
 		xfs_iunlock(ip, XFS_ILOCK_SHARED);
 		/*
 		 * Truncate can race with writeback since writeback doesn't

fs/xfs/xfs_iomap.c

@@ -655,6 +655,7 @@ xfs_iomap_write_allocate(
 	unsigned int	*cow_seq)
 {
 	xfs_mount_t	*mp = ip->i_mount;
+	struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, whichfork);
 	xfs_fileoff_t	offset_fsb, last_block;
 	xfs_fileoff_t	end_fsb, map_start_fsb;
 	xfs_filblks_t	count_fsb;
@@ -768,7 +769,7 @@ xfs_iomap_write_allocate(
 				goto error0;
 
 			if (whichfork == XFS_COW_FORK)
-				*cow_seq = XFS_IFORK_PTR(ip, whichfork)->if_seq;
+				*cow_seq = READ_ONCE(ifp->if_seq);
 			xfs_iunlock(ip, XFS_ILOCK_EXCL);
 		}