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>

3 files changed, 21 insertions, 6 deletions

diff --git a/fs/xfs/libxfs/xfs_iext_tree.c b/fs/xfs/libxfs/xfs_iext_tree.c
index 8a7aea041ee1..771dd072015d 100644
--- a/fs/xfs/libxfs/xfs_iext_tree.c
+++ b/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;
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 235b4ddcd324..49f5f5896a43 100644
--- a/fs/xfs/xfs_aops.c
+++ b/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
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 3282575e2df4..6320aca39f39 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/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);
                 }