1 files changed, 19 insertions, 0 deletions

diff --git a/fs/locks.c b/fs/locks.c
index 242328e17f32..53b0cd153202 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1698,6 +1698,25 @@ out:
  * If the filesystem defines a private ->lock() method, then @conf will
  * be left unchanged; so a caller that cares should initialize it to
  * some acceptable default.
+ *
+ * To avoid blocking kernel daemons, such as lockd, that need to acquire POSIX
+ * locks, the ->lock() interface may return asynchronously, before the lock has
+ * been granted or denied by the underlying filesystem, if (and only if)
+ * fl_grant is set. Callers expecting ->lock() to return asynchronously
+ * will only use F_SETLK, not F_SETLKW; they will set FL_SLEEP if (and only if)
+ * the request is for a blocking lock. When ->lock() does return asynchronously,
+ * it must return -EINPROGRESS, and call ->fl_grant() when the lock
+ * request completes.
+ * If the request is for non-blocking lock the file system should return
+ * -EINPROGRESS then try to get the lock and call the callback routine with
+ * the result. If the request timed out the callback routine will return a
+ * nonzero return code and the file system should release the lock. The file
+ * system is also responsible to keep a corresponding posix lock when it
+ * grants a lock so the VFS can find out which locks are locally held and do
+ * the correct lock cleanup when required.
+ * The underlying filesystem must not drop the kernel lock or call
+ * ->fl_grant() before returning to the caller with a -EINPROGRESS
+ * return code.
  */
 int vfs_lock_file(struct file *filp, unsigned int cmd, struct file_lock *fl, struct file_lock *conf)
 {