linux-kernel - Re: [RESEND PATCH] kernfs: Avoid re-adding kernfs_node into kernfs_notify

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Date:   Tue, 5 Jul 2022 12:34:38 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        m.szyprowski@...sung.com, nathan@...nel.org, michael@...le.cc,
        robh@...nel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, guillaume.tucker@...labora.com,
        pmladek@...e.com
Subject: Re: [RESEND PATCH] kernfs: Avoid re-adding kernfs_node into
 kernfs_notify_list.

Hello,

On Wed, Jul 06, 2022 at 06:18:28AM +1000, Imran Khan wrote:
> In this case, the point of using llist would be to avoid taking the locks in
> consumer.

Given that the consumer can dispatch the whole list, I doubt that's worth
the complication.

> Hmm. My idea was that eventually we will never run into situation where multiple
> producers will end up adding the same node because as soon as first producer
> adds the node (the other potential adders are spinning on kernfs_notify_lock),
> kn->attr.notif_next.next will get a non-NULL value and checking
> (kn->attr.notify_next.next != NULL) will avoid the node getting re-added.

So, here, I don't see how llist can be used without a surrounding lock and I
don't see much point in using llist if we need to use a lock anyway. If this
needs to be made scalable, we need a different strategy (e.g. per-cpu lock /
pending list can be an option).

I'm a bit swamped with other stuff and will likely be less engaged from now
on. I'll try to review patches where possible.

Thanks.

-- 
tejun