vfs: add a retry_estale helper function to handle retries on ESTALE

This function is expected to be called from path-based syscalls to help
them decide whether to try the lookup and call again in the event that
they got an -ESTALE return back on an earier try.

Currently, we only retry the call once on an ESTALE error, but in the
event that we decide that that's not enough in the future, we should be
able to change the logic in this helper without too much effort.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/namei.h

@@ -98,4 +98,20 @@ static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
 	((char *) name)[min(len, maxlen)] = '\0';
 }
 
+/**
+ * retry_estale - determine whether the caller should retry an operation
+ * @error: the error that would currently be returned
+ * @flags: flags being used for next lookup attempt
+ *
+ * Check to see if the error code was -ESTALE, and then determine whether
+ * to retry the call based on whether "flags" already has LOOKUP_REVAL set.
+ *
+ * Returns true if the caller should try the operation again.
+ */
+static inline bool
+retry_estale(const long error, const unsigned int flags)
+{
+	return error == -ESTALE && !(flags & LOOKUP_REVAL);
+}
+
 #endif /* _LINUX_NAMEI_H */