[PATCH] kernfs: Avoid re-adding kernfs_node into kernfs_notify_list.

[Imran Khan <imran.f.khan@oracle.com>]

Kick fsnotify only if an event is not already scheduled for target
kernfs node. commit b8f35fa1188b ("kernfs: Change kernfs_notify_list to
llist.") changed kernfs_notify_list to a llist.
Prior to this list was a singly linked list, protected by
kernfs_notify_lock. Whenever a kernfs_node was added to the list
its ->attr.notify_next was set to head of the list and upon removal
->attr.notify_next was reset to NULL. Addition to kernfs_notify_list
would only happen if kernfs_node was not already in the list i.e.
if ->attr.notify_next was NULL. commit b8f35fa1188b ("kernfs: Change
kernfs_notify_list to llist.") removed this checking and this was wrong
as it resulted in multiple additions for same kernfs_node.

So far this bug only got reflected with some console related setting.
Nathan found this issue when console was specified both in DT and in
kernel command line and Marek found this issue when earlycon was enabled.

This patch avoids adding an already added kernfs_node into notify list.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: b8f35fa1188b ("kernfs: Change kernfs_notify_list to llist.")
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 fs/kernfs/file.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index bb933221b4bae..e8ec054e11c63 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -917,6 +917,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
 	if (free == NULL)
 		return;
 
+	free->next = NULL;
 	attr = llist_entry(free, struct kernfs_elem_attr, notify_next);
 	kn = attribute_to_node(attr, struct kernfs_node, attr);
 	root = kernfs_root(kn);
@@ -992,9 +993,11 @@ void kernfs_notify(struct kernfs_node *kn)
 	rcu_read_unlock();
 
 	/* schedule work to kick fsnotify */
-	kernfs_get(kn);
-	llist_add(&kn->attr.notify_next, &kernfs_notify_list);
-	schedule_work(&kernfs_notify_work);
+	if (kn->attr.notify_next.next != NULL) {
+		kernfs_get(kn);
+		llist_add(&kn->attr.notify_next, &kernfs_notify_list);
+		schedule_work(&kernfs_notify_work);
+	}
 }
 EXPORT_SYMBOL_GPL(kernfs_notify);
 

base-commit: 6cc11d2a1759275b856e464265823d94aabd5eaf
-- 
2.30.2