On 27/6/24 19:54, Jan Kara wrote:
On Thu 27-06-24 09:11:14, Ian Kent wrote:
On 27/6/24 04:47, Matthew Wilcox wrote:
On Wed, Jun 26, 2024 at 04:07:49PM -0400, Lucas Karpinski wrote:
+++ b/fs/namespace.c
@@ -78,6 +78,7 @@ static struct kmem_cache *mnt_cache __ro_after_init;
static DECLARE_RWSEM(namespace_sem);
static HLIST_HEAD(unmounted); /* protected by namespace_sem */
static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
+static bool lazy_unlock = false; /* protected by namespace_sem */
That's a pretty ugly way of doing it. How about this?
Ha!
That was my original thought but I also didn't much like changing all the
callers.
I don't really like the proliferation of these small helper functions either
but if everyone
is happy to do this I think it's a great idea.
So I know you've suggested removing synchronize_rcu_expedited() call in
your comment to v2. But I wonder why is it safe? I *thought*
synchronize_rcu_expedited() is there to synchronize the dropping of the
last mnt reference (and maybe something else) - see the comment at the
beginning of mntput_no_expire() - and this change would break that?
Interesting, because of the definition of lazy umount I didn't look closely
enough at that.
But I wonder, how exactly would that race occur, is holding the rcu read
lock
sufficient since the rcu'd mount free won't be done until it's released (at
least I think that's how rcu works).
In this case, when lazy is true, the mount will have been detached in
umount_tree()
and mnt->mnt_ns set to NULL under the namespace sem write lock. So that
condition
in mntput_no_expre() won't be true and the mount will no longer be found
by the VFS.
I guess the question then becomes will any outstanding lockless path
walks race with
this with only the rcu read lock to protect it, Christian?
Ian
