linux-kernel - Re: [PATCH 4/8] sched,rcu,tracing: Mark preempt_count

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Message-ID: <20200212153823.GW14897@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Feb 2020 16:38:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com
Subject: Re: [PATCH 4/8] sched,rcu,tracing: Mark preempt_count_{add,sub}()
 notrace

On Wed, Feb 12, 2020 at 10:14:15AM -0500, Steven Rostedt wrote:
> > My initial patch has __preempt_count_add/sub() in, but then I figured
> > someone would go complain the tracepoint would go missing.
> 
> Fine, but what bug are you trying to fix? I haven't seen one mentioned
> yet. Function tracing has recursion protection, and tracing
> preempt_count in nmi_enter() causes no problems. What's the problem you
> are trying to solve?

The same one as yesterday; if we hit a tracer in NMI context, when
!rcu_is_watching() and in_nmi() is still 0, we're fucked.