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

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Date:   Fri, 24 May 2019 08:57:44 -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 14:27:24 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > Believe me, I rather not have that array, but I couldn't come up with a
> > better solution to handle freeing of fgraph_ops.  
> 
> The trivial answer would be to refcount the thing, but can't we make
> rcu_tasks do this?

But wouldn't refcounts require atomic operations, something that would
be excruciatingly slow for something that runs on all functions.

rcu_tasks doesn't cross voluntary sleeps, which this does.

> 
> And delay the unreg until all active users are gone -- who gives a crap
> that can take a while.

It could literally be forever (well, until the machine reboots). And
something that could appear to be a memory leak, although a very slow
one. But probably be hard to have more than the number of tasks on the
system.

-- Steve