autofs: add comment about autofs_mountpoint_changed()

The function autofs_mountpoint_changed() is unusual, add a comment about
two cases for which it is needed.

Link: https://lkml.kernel.org/r/165724459804.30914.10974834416046555127.stgit@donald.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/autofs/root.c b/fs/autofs/root.c
index e0fa71e..ca03c1c 100644 (file)
--- a/fs/autofs/root.c
+++ b/fs/autofs/root.c
@@ -291,9 +291,26 @@ static struct dentry *autofs_mountpoint_changed(struct path *path)
        struct dentry *dentry = path->dentry;
        struct autofs_sb_info *sbi = autofs_sbi(dentry->d_sb);
 
-       /*
-        * If this is an indirect mount the dentry could have gone away
-        * as a result of an expire and a new one created.
+       /* If this is an indirect mount the dentry could have gone away
+        * and a new one created.
+        *
+        * This is unusual and I can't remember the case for which it
+        * was originally added now. But an example of how this can
+        * happen is an autofs indirect mount that has the "browse"
+        * option set and also has the "symlink" option in the autofs
+        * map entry. In this case the daemon will remove the browse
+        * directory and create a symlink as the mount leaving the
+        * struct path stale.
+        *
+        * Another not so obvious case is when a mount in an autofs
+        * indirect mount that uses the "nobrowse" option is being
+        * expired at the same time as a path walk. If the mount has
+        * been umounted but the mount point directory seen before
+        * becoming unhashed (during a lockless path walk) when a stat
+        * family system call is made the mount won't be re-mounted as
+        * it should. In this case the mount point that's been removed
+        * (by the daemon) will be stale and the a new mount point
+        * dentry created.
         */
        if (autofs_type_indirect(sbi->type) && d_unhashed(dentry)) {
                struct dentry *parent = dentry->d_parent;