[PATCH 6/7] kernfs: Allow kernfs nodes to be deactivated and re-activated

Tejun Heo
2022-08-20T00:06:46Z

Currently, kernfs nodes can be created deactivated and activated later to
allow creation of multiple nodes to succeed or fail as a group. Extend this
capability so that kernfs nodes can be deactivated and re-activated anytime
and however many times. This can be used to toggle interface files for
features which can be dynamically turned on and off.

kernfs_activate()'s skip conditions are updated so that it doesn't ignore
re-activations and suppress re-activations of files which are being removed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
fs/kernfs/dir.c | 89 ++++++++++++++++++++++++++++++------------
include/linux/kernfs.h | 2 +
2 files changed, 65 insertions(+), 26 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index f857731598cd..6db031362585 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -483,7 +483,7 @@ static bool kernfs_drain(struct kernfs_node *kn)
* worrying about draining.
*/
if (atomic_read(&kn->active) == KN_DEACTIVATED_BIAS &&
- kernfs_should_drain_open_files(kn))
+ !kernfs_should_drain_open_files(kn))
return false;

up_write(&root->kernfs_rwsem);
@@ -1321,14 +1321,15 @@ static struct kernfs_node *kernfs_next_descendant_post(struct kernfs_node *pos,
}

/**
- * kernfs_activate - activate a node which started deactivated
+ * kernfs_activate - activate a node's subtree
* @kn: kernfs_node whose subtree is to be activated
*
- * If the root has KERNFS_ROOT_CREATE_DEACTIVATED set, a newly created node
- * needs to be explicitly activated. A node which hasn't been activated
- * isn't visible to userland and deactivation is skipped during its
- * removal. This is useful to construct atomic init sequences where
- * creation of multiple nodes should either succeed or fail atomically.
+ * If newly created on a root w/ %KERNFS_ROOT_CREATE_DEACTIVATED or after a
+ * kernfs_deactivate() call, @kn is deactivated and invisible to userland. This
+ * function activates all nodes in @kn's inclusive subtree making them visible.
+ *
+ * %KERNFS_ROOT_CREATE_DEACTIVATED is useful when constructing init sequences
+ * where creation of multiple nodes should either succeed or fail atomically.
*
* The caller is responsible for ensuring that this function is not called
* after kernfs_remove*() is invoked on @kn.
@@ -1342,7 +1343,7 @@ void kernfs_activate(struct kernfs_node *kn)

pos = NULL;
while ((pos = kernfs_next_descendant_post(pos, kn))) {
- if (pos->flags & KERNFS_ACTIVATED)
+ if (kernfs_active(pos) || (kn->flags & KERNFS_REMOVING))
continue;

WARN_ON_ONCE(pos->parent && RB_EMPTY_NODE(&pos->rb));
@@ -1355,6 +1356,58 @@ void kernfs_activate(struct kernfs_node *kn)
up_write(&root->kernfs_rwsem);
}

+static void kernfs_deactivate_locked(struct kernfs_node *kn, bool removing)
+{
+ struct kernfs_root *root = kernfs_root(kn);
+ struct kernfs_node *pos;
+
+ lockdep_assert_held_write(&root->kernfs_rwsem);
+
+ /* prevent any new usage under @kn by deactivating all nodes */
+ pos = NULL;
+ while ((pos = kernfs_next_descendant_post(pos, kn))) {
+ if (kernfs_active(pos))
+ atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
+ if (removing)
+ pos->flags |= KERNFS_REMOVING;
+ }
+
+ /*
+ * No new active usage can be created. Drain existing ones. As
+ * kernfs_drain() may drop kernfs_rwsem temporarily, pin @pos so that it
+ * doesn't go away underneath us.
+ *
+ * If kernfs_rwsem was released, restart from the beginning. Forward
+ * progress is guaranteed as a drained node is guaranteed to stay
+ * drained. In the unlikely case that the loop restart ever becomes a
+ * problem, we should be able to work around by batching up the
+ * draining.
+ */
+ pos = NULL;
+ while ((pos = kernfs_next_descendant_post(pos, kn))) {
+ kernfs_get(pos);
+ if (kernfs_drain(pos))
+ pos = NULL;
+ kernfs_put(pos);
+ }
+}
+
+/**
+ * kernfs_deactivate - deactivate a node's subtree
+ * @kn: kernfs_node whose subtree is to be deactivated
+ *
+ * Deactivate @kn's inclusive subtree. On return, the subtree is invisible to
+ * userland and there are no in-flight file operations.
+ */
+void kernfs_deactivate(struct kernfs_node *kn)
+{
+ struct kernfs_root *root = kernfs_root(kn);
+
+ down_write(&root->kernfs_rwsem);
+ kernfs_deactivate_locked(kn, false);
+ up_write(&root->kernfs_rwsem);
+}
+
static void __kernfs_remove(struct kernfs_node *kn)
{
struct kernfs_node *pos;
@@ -1374,26 +1427,12 @@ static void __kernfs_remove(struct kernfs_node *kn)

pr_debug("kernfs %s: removing\n", kn->name);

- /* prevent any new usage under @kn by deactivating all nodes */
- pos = NULL;
- while ((pos = kernfs_next_descendant_post(pos, kn)))
- if (kernfs_active(pos))
- atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
+ kernfs_deactivate_locked(kn, true);

- /* deactivate and unlink the subtree node-by-node */
+ /* unlink the subtree node-by-node */
do {
pos = kernfs_leftmost_descendant(kn);

- /*
- * kernfs_drain() may drop kernfs_rwsem temporarily and @pos's
- * base ref could have been put by someone else by the time
- * the function returns. Make sure it doesn't go away
- * underneath us.
- */
- kernfs_get(pos);
-
- kernfs_drain(pos);
-
/*
* kernfs_unlink_sibling() succeeds once per node. Use it
* to decide who's responsible for cleanups.
@@ -1410,8 +1449,6 @@ static void __kernfs_remove(struct kernfs_node *kn)

kernfs_put(pos);
}
-
- kernfs_put(pos);
} while (pos != kn);
}

diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 367044d7708c..657eea1395b6 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -112,6 +112,7 @@ enum kernfs_node_flag {
KERNFS_SUICIDED = 0x0800,
KERNFS_EMPTY_DIR = 0x1000,
KERNFS_HAS_RELEASE = 0x2000,
+ KERNFS_REMOVING = 0x4000,
};

/* @flags for kernfs_create_root() */
@@ -429,6 +430,7 @@ struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
const char *name,
struct kernfs_node *target);
void kernfs_activate(struct kernfs_node *kn);
+void kernfs_deactivate(struct kernfs_node *kn);
void kernfs_remove(struct kernfs_node *kn);
void kernfs_break_active_protection(struct kernfs_node *kn);
void kernfs_unbreak_active_protection(struct kernfs_node *kn);
--
2.37.2