[PATCH 2/3] dcache: wait for a racing eviction of a secondary root

[Christian Brauner <brauner@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

Commit e9895609cb7f ("wind ->s_roots via ->d_sib instead of ->d_hash")
keeps a secondary root on ->s_roots until dentry_unlist() so that
shrink_dcache_for_umount() can find one that another thread is evicting
and wait for it. But do_one_tree() takes every root off the list before
its dput() which breaks that:

  thread A                              umount
  dentry_kill(root), refcount is 0
    lock_for_kill()
      spin_trylock(->i_lock) fails
      spin_unlock(->d_lock)
                                        shrink_dcache_for_umount()
                                          dget_dlock(root)
                                          do_one_tree(root)
                                            unlink_secondary_root(root)
      spin_lock(->i_lock)
                                            dput(root), refcount is 0
                                              lock_for_kill()
                                                trylock fails, waits
      spin_lock(->d_lock)
      refcount is 0 again, go on
    lockref_mark_dead()
    dentry_unlink_inode()
      drops both locks
                                                root is dead, give up
                                          ->s_roots is empty, done
                                        evict_inodes()
                                        "Busy inodes after unmount"
      iput()

Thread A evicts without pinning the superblock. namespace_unlock() does
that for old mountpoints after dropping namespace_sem, and a secondary
root goes with its last child.

So only take busy roots off the list. Leave the others to
dentry_unlist() so that shrink_dcache_for_umount() finds them and waits
for an eviction in progress. A root that becomes busy after the check is
picked up again and taken off the list then.

Fixes: 78db93943210 ("dcache: keep shrink_dcache_for_umount() making progress on busy roots")
Cc: stable@vger.kernel.org # v7.2+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/dcache.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 72b8e0adab0d..c1d8b398b2d1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1817,8 +1817,13 @@ static void do_one_tree(struct dentry *dentry)
 	d_walk(dentry, dentry, umount_check);
 	spin_lock(&dentry->d_lock);
 	__d_drop(dentry);
-	/* A busy root survives the dput() below so don't leave it on ->s_roots. */
-	if (unlikely(!hlist_unhashed(&dentry->d_sib)))
+	/*
+	 * A busy root survives the dput() below so don't leave it on ->s_roots.
+	 * An idle one stays until dentry_unlist() so that umount can wait for
+	 * a racing eviction, see lock_for_kill().
+	 */
+	if (unlikely(dentry->d_lockref.count > 1 &&
+		     !hlist_unhashed(&dentry->d_sib)))
 		unlink_secondary_root(dentry);
 	spin_unlock(&dentry->d_lock);
 	dput(dentry);

-- 
2.53.0