linux-kernel - Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

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Message-ID: <CAEXW_YT_vh1iGOwXdymzPO4-k59=+tqQCWHHokv+Kfif5GHypw@mail.gmail.com>
Date:   Tue, 30 Jul 2019 00:19:36 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Eiichi Tsukata <devel@...ukata.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Luto <luto@...capital.net>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

On Mon, Jul 29, 2019 at 10:15 PM Steven Rostedt <rostedt@...dmis.org> wrote:
[snip]
> > If the problem was only with userstacktrace, it will be reasonable to
> > surround only the userstack unwinder. But the situation is similar to
> > the previous "tracing vs CR2" case. As Peter taught me in
> > https://lore.kernel.org/lkml/20190708074823.GV3402@hirez.programming.kicks-ass.net/
> > there are some other codes likely to to user access.
> > So I surround preemptirq events earlier.
>
> I disagree. The issue is with the attached callbacks that call
> something (a stack unwinder) that can fault.
>
> This is called whenever irqs are disabled. I say we surround the
> culprit (the stack unwinder or stack trace) and not punish the irq
> enable/disable events.

I agree with everything Steve said.

thanks,

 - Joel