[PATCH v2 1/3] NFS: don't let delegated c/mtime stamps go backward

[Jeff Layton <jlayton@kernel.org>]

With a timestamp delegation the client stamps c/mtime from its own
coarse clock. That can land before the server's fine-grained ctime the
client cached from the OPEN or SETATTR reply, e.g. on a WRITE right
after an exclusive create. The server then either rejects the times
(they don't pass dl_ctime) or can't apply the ctime, since
inode_set_ctime_deleg() never moves ctime backward. Either way the
c/mtime the client reported while holding the delegation changes
after DELEGRETURN; git t1092 over NFSv4.2 sees a stat-dirty file.

When a delegated stamp isn't after the cached ctime, use the cached
ctime + 1ns instead. Compute the stamp before storing it, so lockless
readers (getattr, CB_GETATTR) never see the trailing coarse time.

Fixes: e12912d94137 ("NFSv4: Add support for delegated atime and mtime attributes")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfs/inode.c | 40 ++++++++++++++++++++++++++--------------
 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 9839087cf373..98d8a34fab9d 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -629,25 +629,33 @@ nfs_fattr_fixup_delegated(struct inode *inode, struct nfs_fattr *fattr)
 	}
 }
 
+/*
+ * With delegated timestamps, the client's coarse clock can trail the
+ * server's fine-grained ctime cached from an OPEN or SETATTR reply. The
+ * server won't move ctime backward when the times are returned, so a
+ * new stamp must land after the cached ctime.
+ */
+static struct timespec64 nfs_delegated_stamp(struct inode *inode)
+{
+	struct timespec64 ctime = inode_get_ctime(inode);
+	struct timespec64 now = current_time(inode);
+
+	if (timespec64_compare(&now, &ctime) <= 0) {
+		now = ctime;
+		timespec64_add_ns(&now, 1);
+	}
+	return now;
+}
+
 static void nfs_set_timestamps_to_ts(struct inode *inode, struct iattr *attr)
 {
 	unsigned int cache_flags = 0;
 
 	if (attr->ia_valid & ATTR_MTIME_SET) {
-		struct timespec64 ctime = inode_get_ctime(inode);
-		struct timespec64 mtime = inode_get_mtime(inode);
-		struct timespec64 now;
-		bool updated = false;
-
-		now = inode_set_ctime_current(inode);
-		if (!timespec64_equal(&now, &ctime))
-			updated = true;
-
+		/* The stamp is always after the old ctime */
+		inode_set_ctime_to_ts(inode, nfs_delegated_stamp(inode));
 		inode_set_mtime_to_ts(inode, attr->ia_mtime);
-		if (!timespec64_equal(&now, &mtime))
-			updated = true;
-
-		inode_maybe_inc_iversion(inode, updated);
+		inode_maybe_inc_iversion(inode, true);
 		cache_flags |= NFS_INO_INVALID_CTIME | NFS_INO_INVALID_MTIME;
 	}
 	if (attr->ia_valid & ATTR_ATIME) {
@@ -665,7 +673,11 @@ static void nfs_update_atime(struct inode *inode)
 
 static void nfs_update_mtime(struct inode *inode)
 {
-	inode_update_time(inode, FS_UPD_CMTIME, 0);
+	struct timespec64 now = nfs_delegated_stamp(inode);
+
+	/* Not inode_update_time(): lockless readers would see its stamp */
+	inode_set_ctime_to_ts(inode, now);
+	inode_set_mtime_to_ts(inode, now);
 	NFS_I(inode)->cache_validity &=
 		~(NFS_INO_INVALID_CTIME | NFS_INO_INVALID_MTIME);
 }

-- 
2.56.0