linux-kernel - Re: [RFC][PATCH 03/14 v2] function_graph: Allow multiple users to attach to function graph

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Message-ID: <20190524081219.25de03f6@gandalf.local.home>
Date:   Fri, 24 May 2019 08:12:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC][PATCH 03/14 v2] function_graph: Allow multiple users to
 attach to function graph

On Fri, 24 May 2019 13:26:08 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> But but but but.. why not add all the required bits to the shadow stack
> in the first place and do away with the array entirely?

What required bits would that be? The pointer to the fgraph_ops,
because we need that to pass to the calling function.

> 
> So on ret, just keep POP'ing until either the stack is empty or the
> entry is for another function.

When we hit a fgraph_ops, how do we know if it was freed or not? We
can't just blindly reference it.

The idea of the array, is that we can maintain state in a single
location of when the fgraph_ops is freed. If we return from a function,
we have an index and a counter, and if the counter doesn't match with
what's in the array, then we know that the fgraph_ops is no longer
around and we just drop it.

The reason for the array, is to keep track of if the fgraph_ops has
been freed or not. Otherwise, when we unregister the fgraph_ops, we
would need to search all shadow stacks, looking for it to unreference
it.

Believe me, I rather not have that array, but I couldn't come up with a
better solution to handle freeing of fgraph_ops.

-- Steve