Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 18 Dec 2020 16:33:56 +0100
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, Sven Schnelle <svens@linux.ibm.com>
Subject: Re: __local_bh_enable_ip() vs lockdep
Message-ID: <20201218153356.GA7932@osiris>
References: <20201215190152.GA22285@osiris>
 <20201215144724.40ab7612@gandalf.local.home>
 <20201216175259.GP3040@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20201216175259.GP3040@hirez.programming.kicks-ass.net>
Precedence: bulk

On Wed, Dec 16, 2020 at 06:52:59PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 15, 2020 at 02:47:24PM -0500, Steven Rostedt wrote:
> > On Tue, 15 Dec 2020 20:01:52 +0100
> > Heiko Carstens <hca@linux.ibm.com> wrote:
> > 
> > > Hello,
> > > 
> > > the ftrace stack tracer kernel selftest is able to trigger the warning
> > > below from time to time. This looks like there is an ordering problem
> > > in __local_bh_enable_ip():
> > > first there is a call to lockdep_softirqs_on() and afterwards
> > > preempt_count_sub() is ftraced before it was able to modify
> > > preempt_count:
> > 
> > Don't run ftrace stack tracer when debugging lockdep. ;-)
> > 
> >   /me runs!
> 
> Ha!, seriously though; that seems like something we've encountered
> before, but my google-fu is failing me.
> 
> Do you remember what, if anything, was the problem with this?
> 
> ---
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index d5bfd5e661fc..9d71046ea247 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -186,7 +186,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
>  	 * Keep preemption disabled until we are done with
>  	 * softirq processing:
>  	 */
> -	preempt_count_sub(cnt - 1);
> +	__preempt_count_sub(cnt - 1);
>  
>  	if (unlikely(!in_interrupt() && local_softirq_pending())) {
>  		/*

FWIW,

Tested-by: Heiko Carstens <hca@linux.ibm.com>

Peter, will you make proper patch out of this?