1 files changed, 9 insertions, 2 deletions

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 03afce51709..4ea7ea53442 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1628,8 +1628,15 @@ The difference to vc-do-command is that this function always invokes
          (or coding-system-for-read vc-git-log-output-coding-system))
         (coding-system-for-write
          (or coding-system-for-write vc-git-commits-coding-system))
-        (process-environment (cons "PAGER=" process-environment)))
-    (push "GIT_DIR" process-environment)
+        (process-environment
+         (append
+          `("GIT_DIR"
+            "PAGER="
+            ;; Avoid repository locking during background operations
+            ;; (bug#21559).
+            ,@(when revert-buffer-in-progress-p
+                '("GIT_OPTIONAL_LOCKS=0")))
+          process-environment)))
     (apply 'process-file vc-git-program nil buffer nil command args)))
 
 (defun vc-git--out-ok (command &rest args)