nfsd4: factor ctime into change attribute

Factoring ctime into the nfsv4 change attribute gives us better
properties than just i_version alone.

Eventually we'll likely also expose this (as opposed to raw i_version)
to userspace, at which point we'll want to move it to a common helper,
called from either userspace or individual filesystems. For now, nfsd
is the only user.

Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs3xdr.c

···
 		printk("nfsd: inode locked twice during operation.\n");
 
 	err = fh_getattr(fhp, &fhp->fh_post_attr);
-	fhp->fh_post_change = d_inode(fhp->fh_dentry)->i_version;
+	fhp->fh_post_change = nfsd4_change_attribute(d_inode(fhp->fh_dentry));
 	if (err) {
 		fhp->fh_post_saved = false;
 		/* Grab the ctime anyway - set_change_info might use it */

fs/nfsd/nfs4xdr.c

···
 		*p++ = cpu_to_be32(convert_to_wallclock(exp->cd->flush_time));
 		*p++ = 0;
 	} else if (IS_I_VERSION(inode)) {
-		p = xdr_encode_hyper(p, inode->i_version);
+		p = xdr_encode_hyper(p, nfsd4_change_attribute(inode));
 	} else {
 		*p++ = cpu_to_be32(stat->ctime.tv_sec);
 		*p++ = cpu_to_be32(stat->ctime.tv_nsec);

fs/nfsd/nfsfh.h

···
 }
 
 /*
+ * We could use i_version alone as the change attribute.  However,
+ * i_version can go backwards after a reboot.  On its own that doesn't
+ * necessarily cause a problem, but if i_version goes backwards and then
+ * is incremented again it could reuse a value that was previously used
+ * before boot, and a client who queried the two values might
+ * incorrectly assume nothing changed.
+ *
+ * By using both ctime and the i_version counter we guarantee that as
+ * long as time doesn't go backwards we never reuse an old value.
+ */
+static inline u64 nfsd4_change_attribute(struct inode *inode)
+{
+	u64 chattr;
+
+	chattr =  inode->i_ctime.tv_sec;
+	chattr <<= 30;
+	chattr += inode->i_ctime.tv_nsec;
+	chattr += inode->i_version;
+	return chattr;
+}
+
+/*
  * Fill in the pre_op attr for the wcc data
  */
 static inline void
···
 		fhp->fh_pre_mtime = inode->i_mtime;
 		fhp->fh_pre_ctime = inode->i_ctime;
 		fhp->fh_pre_size  = inode->i_size;
-		fhp->fh_pre_change = inode->i_version;
+		fhp->fh_pre_change = nfsd4_change_attribute(inode);
 		fhp->fh_pre_saved = true;
 	}
 }