linux-kernel - Re: [PATCH v2 2/3] tracing/fprobe: Support raw tracepoint events on modules

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Date: Tue, 4 Jun 2024 12:34:18 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, don
 <zds100@...il.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] tracing/fprobe: Support raw tracepoint events on
 modules

On Tue, 4 Jun 2024 11:02:16 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> I see.
> 
> It looks like there are a few things we could improve there:
> 
> 1) With your approach, modules need to be already loaded before
> attaching an fprobe event to them. This effectively prevents
> attaching to any module init code. Is there any way we could allow
> this by implementing a module coming notifier in fprobe as well ?
> This would require that fprobes are kept around in a data structure
> that matches the modules when they are loaded in the coming notifier.

The above sounds like a nice enhancement, but not something necessary for
this series.

> 
> 2) Given that the fprobe module going notifier is protected by the
> event_mutex, can we use locking rather than reference counting
> in fprobe attach to guarantee the target module is not reclaimed
> concurrently ? This would remove the transient side-effect of
> holding a module reference count which temporarily prevents module
> unload.

Why do we care about unloading modules during the transition? Note, module
unload has always been considered a second class citizen, and there's been
talks in the past to even rip it out.

-- Steve