Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount

[Matthew Wilcox <willy@infradead.org>]

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?

+++ b/fs/namespace.c
@@ -1553,7 +1553,7 @@ int may_umount(struct vfsmount *mnt)
 
 EXPORT_SYMBOL(may_umount);
 
-static void namespace_unlock(void)
+static void __namespace_unlock(bool lazy)
 {
        struct hlist_head head;
        struct hlist_node *p;
@@ -1570,7 +1570,8 @@ static void namespace_unlock(void)
        if (likely(hlist_empty(&head)))
                return;
 
-       synchronize_rcu_expedited();
+       if (!lazy)
+               synchronize_rcu_expedited();
 
        hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
                hlist_del(&m->mnt_umount);
@@ -1578,6 +1579,11 @@ static void namespace_unlock(void)
        }
 }
 
+static inline void namespace_unlock(void)
+{
+       __namespace_unlock(false);
+}
+
 static inline void namespace_lock(void)
 {
        down_write(&namespace_sem);
@@ -1798,7 +1804,7 @@ static int do_umount(struct mount *mnt, int flags)
        }
 out:
        unlock_mount_hash();
-       namespace_unlock();
+       __namespace_unlock(flags & MNT_DETACH);
        return retval;
 }
 

(other variants on this theme might be to pass the flags to
__namespace_unlock() and check MNT_DETACH there)