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Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty
- From: Ingo Molnar <mingo at kernel dot org>
- To: Masami Hiramatsu <mhiramat at kernel dot org>
- Cc: Steven Rostedt <rostedt at goodmis dot org>, Ingo Molnar <mingo at redhat dot com>, Alban Crequy <alban dot crequy at gmail dot com>, Alban Crequy <alban at kinvolk dot io>, Alexei Starovoitov <ast at kernel dot org>, Jonathan Corbet <corbet at lwn dot net>, Arnaldo Carvalho de Melo <acme at redhat dot com>, Omar Sandoval <osandov at fb dot com>, linux-doc at vger dot kernel dot org, netdev at vger dot kernel dot org, linux-kernel at vger dot kernel dot org, iago at kinvolk dot io, michael at kinvolk dot io, Dorau Lukasz <lukasz dot dorau at intel dot com>, systemtap at sourceware dot org
- Date: Thu, 30 Mar 2017 08:53:32 +0200
- Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty
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- References: <149076484118.24574.7083269903420611708.stgit@devbox> <149076498222.24574.679546540523044200.stgit@devbox> <20170329063005.GA12220@gmail.com> <20170329172510.e012406497fd38a54d5069b3@kernel.org>
* Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > So this is something I missed while the original code was merged, but the concept
> > looks a bit weird: why do we do any "allocation" while a handler is executing?
> >
> > That's fundamentally fragile. What's the maximum number of parallel
> > 'kretprobe_instance' required per kretprobe - one per CPU?
>
> It depends on the place where we put the probe. If the probed function will be
> blocked (yield to other tasks), then we need a same number of threads on
> the system which can invoke the function. So, ultimately, it is same
> as function_graph tracer, we need it for each thread.
So then put it into task_struct (assuming there's no kretprobe-inside-kretprobe
nesting allowed). There's just no way in hell we should be calling any complex
kernel function from kernel probes!
I mean, think about it, a kretprobe can be installed in a lot of places, and now
we want to call get_free_pages() from it?? This would add a massive amount of
fragility.
Instrumentation must be _simple_, every patch that adds more complexity to the
most fundamental code path of it should raise a red flag ...
So let's make this more robust, ok?
Thanks,
Ingo