linux-kernel - Re: [PATCH v4 01/27] lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions

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Message-ID: <20200224101050.GE14897@hirez.programming.kicks-ass.net>
Date:   Mon, 24 Feb 2020 11:10:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, paulmck@...nel.org,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, luto@...nel.org, tony.luck@...el.com,
        frederic@...nel.org, dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v4 01/27] lockdep: Teach lockdep about "USED" <- "IN-NMI"
 inversions

On Fri, Feb 21, 2020 at 10:08:43PM -0500, Joel Fernandes wrote:
> On Fri, Feb 21, 2020 at 02:34:17PM +0100, Peter Zijlstra wrote:
> > nmi_enter() does lockdep_off() and hence lockdep ignores everything.
> > 
> > And NMI context makes it impossible to do full IN-NMI tracking like we
> > do IN-HARDIRQ, that could result in graph_lock recursion.
> 
> The patch makes sense to me.
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> NOTE:
> Also, I was wondering if we can detect the graph_lock recursion case and
> avoid doing anything bad, that way we enable more of the lockdep
> functionality for NMI where possible. Not sure if the suggestion makes sense
> though!

Yeah, I considered playing trylock games, but figured I shouldn't make
it more complicated that it needs to be.