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

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Jul 02, 2022 at 12:50:47AM +1000, Imran Khan wrote:
> 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>

This should also include:

Reported-by: Michael Walle <michael@walle.cc>

> 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>

For the ARCH=um case that I noticed:

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  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
>