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From: Masami Hiramatsu <mhiramat@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com>,
	linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org,
	tom.zanussi@linux.intel.com
Subject: Re: [PATCH v4] [RFC] trace: Add kprobe on tracepoint
Date: Fri, 13 Aug 2021 00:06:59 +0900	[thread overview]
Message-ID: <20210813000659.48eafbcfeeaa30adcc8a5363@kernel.org> (raw)
In-Reply-To: <20210812094439.56303efa@oasis.local.home>

On Thu, 12 Aug 2021 09:44:39 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 12 Aug 2021 20:31:10 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > > Yes, anyway we need a way to find loops on histogram/eprobe at last.  
> > 
> > BTW, what about using similar machanism of "current_kprobe()" to detect
> > the reccursion? As an easy way, prepare a static per-cpu pointer which sets
> > the current eprobe and if the eprobe handler detects that is already set,
> > it may warn (or silently ignore) and reject it.
> > (Of course it is better to detect the loop when user sets the hist-trigger
> > by reverse link)
> 
> Thinking more about this, I believe there is a use case for synthetic
> event on a eprobe. Basically:
> 
>   normal_event -> eprobe (extracts struct data into $dat) -> onmax($dat) -> synthetic event
> 
> But I can not come up with any use case of:
> 
>   eprobe -> synthetic event -> eprobe
> 
> or
> 
>   synthetic event -> eprobe -> synthetic event
> 
> That's because once you have an eprobe, you can extract what you want,
> and once you have that synthetic event, you can get the data you want.
> 
> Maybe we should prevent the above and allow one eprobe on a synthetic
> event and one synthetic event on an eprobe.
> 
> Or just don't prevent it at all, and let the user shoot themselves in
> the foot ;-)
> 
> The more I think about this, I'm thinking we just let them shoot
> themselves if they want to.

I agree. Or, at least we can prevent the loop at runtime as I said.
BTW, does synthetic event itself detect and prevent loops? I think
the key point is always synthetic event, so if the loop detector
is implemented, it should be done on the synthetic event.

> 
> But I still agree that eprobes should not be attached to kprobes or
> uprobes directly (although they may be able to be attached to a
> synthetic event that is attached to one!)

Yes.

Thank you,


> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@kernel.org>

  reply	other threads:[~2021-08-12 15:07 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-11 14:14 Tzvetomir Stoyanov (VMware)
2021-08-11 15:03 ` Masami Hiramatsu
2021-08-11 15:22   ` Steven Rostedt
2021-08-12  1:27     ` Masami Hiramatsu
2021-08-12  3:46       ` Steven Rostedt
2021-08-12  9:44         ` Masami Hiramatsu
2021-08-12 11:14           ` Masami Hiramatsu
2021-08-12  4:02       ` Steven Rostedt
2021-08-12 11:15         ` Masami Hiramatsu
2021-08-12 11:31       ` Masami Hiramatsu
2021-08-12 13:44         ` Steven Rostedt
2021-08-12 15:06           ` Masami Hiramatsu [this message]
2021-08-12 15:44 ` Masami Hiramatsu
2021-08-16 21:40   ` Steven Rostedt
2021-08-17 11:52     ` Masami Hiramatsu

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