Re: [RFC PATCH 1/2] kernel/notifier: replace single-linked list with double-linked list for reverse traversal

[Song Chen <chensong_2000@126.com>]

Hi Hiramatsu san,


On 4/20/26 13:44, Masami Hiramatsu (Google) wrote:
> Hi Song,
> 
> On Wed, 15 Apr 2026 15:01:37 +0800
> chensong_2000@189.cn wrote:
> 
>> From: Song Chen <chensong_2000@189.cn>
>>
>> The current notifier chain implementation uses a single-linked list
>> (struct notifier_block *next), which only supports forward traversal
>> in priority order. This makes it difficult to handle cleanup/teardown
>> scenarios that require notifiers to be called in reverse priority order.
> 
> What about introducing a new notification callback API that allows you
> to describe dependencies between callback functions?
> 
> For example, when registering a callback, you could register a string
> as an ID and specify whether to call it before or after that ID,
> or you could register a comparison function that is called when adding
> to a list. (I prefer @name and @depends fields so that it can be easily
> maintained.)
> 
> This would allow for better dependency building when adding to the list.
> 

Is the new notification callback API going to replace 
blocking_notifier_chain in module loader? or an expansion inside 
blocking_notifier_chain but introducing less complexity?
>>
>> A concrete example is the ordering dependency between ftrace and
>> livepatch during module load/unload. see the detail here [1].
> 
> If this only concerns notification callback issues with the ftrace
> and livepatch modules, it's far more robust to simply call the
> necessary processing directly when the modules load and unload,
> rather than registering notification callbacks externally.
> 
> There are fprobe, kprobe and its trace-events, all of them are using
> ftrace as its fundation layer. In this case, I always needs to
> consider callback order when a module is unloaded.
> 
> If ftrace is working as a part of module callbacks, it will conflict
> with fprobe/kprobe module callback. Of course we can reorder it with
> modifying its priority. But this is ugly, because when we introduce
> a new other feature which depends on another layer, we need to
> reorder the callback's priority number on the list.
> 
> Based on the above, I don't think this can be resolved simply by
> changing the list of notification callbacks to a bidirectional list.
> 
> Thank you,
> 

understood, many thanks for your proposal, i will think  about it.

best regards,

Song