--- /n/sources/plan9/sys/src/cmd/unix/u9fs/plan9.h	Mon Sep 17 21:23:57 2012
+++ /sys/src/cmd/unix/u9fs/plan9.h	Tue Aug  4 00:00:00 2026
@@ -45,6 +45,19 @@ typedef unsigned long long int  uint64_t;
 #include <string.h>		/* for memmove */
 #include <unistd.h>		/* for write */
 #include <fcntl.h>
+#include <sys/stat.h>		/* for the mtime fields of struct stat */
+
+/*
+ * st_mtime alone is too coarse to version a qid: a rewrite in place
+ * within the second would keep it, and clients cache contents by qid.
+ */
+#if defined(__APPLE__)
+#define MTIMENSEC(st)	((st)->st_mtimespec.tv_nsec)
+#elif defined(st_mtime)		/* the POSIX 2008 timespec members */
+#define MTIMENSEC(st)	((st)->st_mtim.tv_nsec)
+#else
+#define MTIMENSEC(st)	0
+#endif
 
 #define ulong p9ulong		/* because sys/types.h has some of these sometimes */
 #define ushort p9ushort
--- /n/sources/plan9/sys/src/cmd/unix/u9fs/u9fs.c	Fri Jul 29 22:16:34 2011
+++ /sys/src/cmd/unix/u9fs/u9fs.c	Tue Aug  4 00:00:00 2026
@@ -651,7 +651,7 @@ stat2qid(struct stat *st)
 			*(dev_t*)q ^= st->st_dev;
 	}
 
-	qid.vers = st->st_mtime ^ (st->st_size << 8);
+	qid.vers = MTIMENSEC(st) ^ st->st_mtime ^ (st->st_size << 8);
 	qid.type = modebyte(st);
 	return qid;
 }