From: Petr Mladek <pmladek@suse•com>
To: Masami Hiramatsu <mhiramat@kernel•org>
Cc: chensong_2000@189•cn, rafael@kernel•org, lenb@kernel•org,
mturquette@baylibre•com, sboyd@kernel•org,
viresh.kumar@linaro•org, agk@redhat•com, snitzer@kernel•org,
mpatocka@redhat•com, bmarzins@redhat•com, song@kernel•org,
yukuai@fnnas•com, linan122@huawei•com,
jason.wessel@windriver•com, danielt@kernel•org,
dianders@chromium•org, horms@kernel•org, davem@davemloft•net,
edumazet@google•com, kuba@kernel•org, pabeni@redhat•com,
paulmck@kernel•org, frederic@kernel•org, mcgrof@kernel•org,
petr.pavlu@suse•com, da.gomez@kernel•org,
samitolvanen@google•com, atomlin@atomlin•com,
jpoimboe@kernel•org, jikos@kernel•org, mbenes@suse•cz,
joe.lawrence@redhat•com, rostedt@goodmis•org,
mark.rutland@arm•com, mathieu.desnoyers@efficios•com,
linux-modules@vger•kernel.org, linux-kernel@vger•kernel.org,
linux-trace-kernel@vger•kernel.org, linux-acpi@vger•kernel.org,
linux-clk@vger•kernel.org, linux-pm@vger•kernel.org,
live-patching@vger•kernel.org, dm-devel@lists•linux.dev,
linux-raid@vger•kernel.org, kgdb-bugreport@lists•sourceforge.net,
netdev@vger•kernel.org
Subject: Re: [RFC PATCH 1/2] kernel/notifier: replace single-linked list with double-linked list for reverse traversal
Date: Tue, 21 Apr 2026 11:05:21 +0200 [thread overview]
Message-ID: <aec90caYZDHDAHgw@pathway.suse.cz> (raw)
In-Reply-To: <20260420144429.57b45f2beece690bceea96ec@kernel.org>
On Mon 2026-04-20 14:44:29, Masami Hiramatsu 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 looks too complex. It would make sense only
when this API has more users.
Also this won't be enough for the ftrace/livepatch callbacks.
They need to be ordered against against each other. But they
also need to be called before/after all other callbacks.
For example, when the module is loaded:
+ 1st frace
+ 2nd livepatch
+ then other notifiers
See the commit c1bf08ac26e92122 ("ftrace: Be first to run code
modification on modules").
> This would allow for better dependency building when adding to the list.
> >
> > 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.
I agree. I would keep it as is (hardcoded).
Best Regards,
Petr
next prev parent reply other threads:[~2026-04-21 9:05 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-15 7:01 [RFC PATCH 1/2] kernel/notifier: replace single-linked list with double-linked list for reverse traversal chensong_2000
2026-04-15 7:40 ` Christoph Hellwig
2026-04-16 10:33 ` Petr Mladek
2026-04-19 0:07 ` Song Chen
2026-04-16 12:30 ` David Laight
2026-04-16 14:54 ` Petr Mladek
2026-04-16 19:15 ` David Laight
2026-04-19 0:21 ` Song Chen
2026-04-20 5:44 ` Masami Hiramatsu
2026-04-21 9:05 ` Petr Mladek [this message]
2026-04-26 13:56 ` Song Chen
2026-04-26 14:14 ` Song Chen
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