diff --git a/hgext/fsmonitor/state.py b/hgext/fsmonitor/state.py
--- a/hgext/fsmonitor/state.py
+++ b/hgext/fsmonitor/state.py
@@ -13,7 +13,10 @@ import socket
 import struct
 
 from mercurial.i18n import _
-from mercurial import pathutil
+from mercurial import (
+    pathutil,
+    util,
+)
 
 _version = 4
 _versionformat = ">I"
@@ -24,6 +27,7 @@ class state(object):
         self._ui = repo.ui
         self._rootdir = pathutil.normasprefix(repo.root)
         self._lastclock = None
+        self._identity = util.filestat(None)
 
         self.mode = self._ui.config('fsmonitor', 'mode', default='on')
         self.walk_on_invalidate = self._ui.configbool(
@@ -35,10 +39,13 @@ class state(object):
         try:
             file = self._vfs('fsmonitor.state', 'rb')
         except IOError as inst:
+            self._identity = util.filestat(None)
             if inst.errno != errno.ENOENT:
                 raise
             return None, None, None
 
+        self._identity = util.filestat.fromfp(file)
+
         versionbytes = file.read(4)
         if len(versionbytes) < 4:
             self._ui.log(
@@ -90,8 +97,16 @@ class state(object):
             self.invalidate()
             return
 
+        # Read the identity from the file on disk rather than from the open file
+        # pointer below, because the latter is actually a brand new file.
+        identity = util.filestat.frompath(self._vfs.join('fsmonitor.state'))
+        if identity != self._identity:
+            self._ui.debug('skip updating fsmonitor.state: identity mismatch\n')
+            return
+
         try:
-            file = self._vfs('fsmonitor.state', 'wb', atomictemp=True)
+            file = self._vfs('fsmonitor.state', 'wb', atomictemp=True,
+                checkambig=True)
         except (IOError, OSError):
             self._ui.warn(_("warning: unable to write out fsmonitor state\n"))
             return
@@ -111,6 +126,7 @@ class state(object):
         except OSError as inst:
             if inst.errno != errno.ENOENT:
                 raise
+        self._identity = util.filestat(None)
 
     def setlastclock(self, clock):
         self._lastclock = clock
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1519,6 +1519,11 @@ class filestat(object):
             stat = None
         return cls(stat)
 
+    @classmethod
+    def fromfp(cls, fp):
+        stat = os.fstat(fp.fileno())
+        return cls(stat)
+
     __hash__ = object.__hash__
 
     def __eq__(self, old):
diff --git a/tests/test-dirstate-race.t b/tests/test-dirstate-race.t
--- a/tests/test-dirstate-race.t
+++ b/tests/test-dirstate-race.t
@@ -160,6 +160,34 @@ treated differently in _checklookup() ac
 
   $ rm b
 
+#if fsmonitor
+
+Create fsmonitor state.
+
+  $ hg status
+  $ f --type .hg/fsmonitor.state
+  .hg/fsmonitor.state: file
+
+Test that invalidating fsmonitor state in the middle (which doesn't require the
+wlock) causes the fsmonitor update to be skipped.
+hg debugrebuilddirstate ensures that the dirstaterace hook will be called, but
+it also invalidates the fsmonitor state. So back it up and restore it.
+
+  $ mv .hg/fsmonitor.state .hg/fsmonitor.state.tmp
+  $ hg debugrebuilddirstate
+  $ mv .hg/fsmonitor.state.tmp .hg/fsmonitor.state
+
+  $ cat > $TESTTMP/dirstaterace.sh <<EOF
+  > rm .hg/fsmonitor.state
+  > EOF
+
+  $ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py --debug
+  skip updating fsmonitor.state: identity mismatch
+  $ f .hg/fsmonitor.state
+  .hg/fsmonitor.state: file not found
+
+#endif
+
 Set up a rebase situation for issue5581.
 
   $ echo c2 > a