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Date:   Tue, 25 Aug 2020 15:30:05 +0200
From:   peterz@infradead.org
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     "Eddy_Wu@trendmicro.com" <Eddy_Wu@trendmicro.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: x86/kprobes: kretprobe fails to triggered if kprobe at function
 entry is not optimized (trigger by int3 breakpoint)
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On Tue, Aug 25, 2020 at 10:15:55PM +0900, Masami Hiramatsu wrote:

> > damn... one last problem is dangling instances.. so close.
> > We can apparently unregister a kretprobe while there's still active
> > kretprobe_instance's out referencing it.
> 
> Yeah, kretprobe already provided the per-instance data (as far as
> I know, only systemtap depends on it). We need to provide it for
> such users.
> But if we only have one lock, we can avoid checking NMI because
> we can check the recursion with trylock. It is needed only if the
> kretprobe uses per-instance data. Or we can just pass a dummy
> instance on the stack.

I think it is true in general, you can unregister a rp while tasks are
preempted.

Anyway,. I think I have a solution, just need to talk to paulmck for a
bit.

> > Ignoring that issue for the moment, the below seems to actually work.
> 
> OK, this looks good to me too.
> I'll make a series to rewrite kretprobe based on this patch, OK?

Please, I'll send the fix along when I have it.