lockd: don't attempt blocking locks on nfs reexports

As in the v4 case, it doesn't work well to block waiting for a lock on
an nfs filesystem.

As in the v4 case, that means we're depending on the client to poll.
It's probably incorrect to depend on that, but I *think* clients do poll
in practice.  In any case, it's an improvement over hanging the lockd
thread indefinitely as we currently are.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/lockd/svclock.c

@@ -31,6 +31,7 @@
 #include <linux/lockd/nlm.h>
 #include <linux/lockd/lockd.h>
 #include <linux/kthread.h>
+#include <linux/exportfs.h>
 
 #define NLMDBG_FACILITY		NLMDBG_SVCLOCK
 
@@ -470,18 +471,24 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
 	    struct nlm_cookie *cookie, int reclaim)
 {
 	struct nlm_block	*block = NULL;
+	struct inode		*inode = nlmsvc_file_inode(file);
 	int			error;
 	int			mode;
+	int			async_block = 0;
 	__be32			ret;
 
 	dprintk("lockd: nlmsvc_lock(%s/%ld, ty=%d, pi=%d, %Ld-%Ld, bl=%d)\n",
-				nlmsvc_file_inode(file)->i_sb->s_id,
-				nlmsvc_file_inode(file)->i_ino,
+				inode->i_sb->s_id, inode->i_ino,
 				lock->fl.fl_type, lock->fl.fl_pid,
 				(long long)lock->fl.fl_start,
 				(long long)lock->fl.fl_end,
 				wait);
 
+	if (inode->i_sb->s_export_op->flags & EXPORT_OP_SYNC_LOCKS) {
+		async_block = wait;
+		wait = 0;
+	}
+
 	/* Lock file against concurrent access */
 	mutex_lock(&file->f_mutex);
 	/* Get existing block (in case client is busy-waiting)
@@ -542,7 +549,7 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
 			 */
 			if (wait)
 				break;
-			ret = nlm_lck_denied;
+			ret = async_block ? nlm_lck_blocked : nlm_lck_denied;
 			goto out;
 		case FILE_LOCK_DEFERRED:
 			if (wait)