linux-kernel - Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt

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Date:   Wed, 29 May 2019 15:49:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, williams@...hat.com,
        daniel@...stot.me, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yangtao Li <tiny.windzz@...il.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping
 due to a preempt_counter change

On Wed, May 29, 2019 at 09:42:13AM -0400, Steven Rostedt wrote:
> > And the preempt_irqoff tracer had better also consume the IRQ events,
> > and if it does that it can DTRT without extra bits on, even with that
> > race.
> > 
> > Consider:
> > 
> > 	preempt_disable()
> > 	  preempt_count += 1;
> > 	  <IRQ>
> > 	    trace_irq_enter();
> > 
> > 	    trace_irq_exit();
> > 	  </IRQ>
> > 	  trace_preempt_disable();
> > 
> > 	/* does stuff */
> > 
> > 	preempt_enable()
> > 	  preempt_count -= 1;
> > 	  trace_preempt_enable();
> > 
> > You're saying preempt_irqoff() fails to connect the two because of the
> > hole between trace_irq_exit() and trace_preempt_disable() ?
> > 
> > But trace_irq_exit() can see the raised preempt_count and set state
> > for trace_preempt_disable() to connect.
> 
> That's basically what I was suggesting as the solution to this ;-)

You were wanting changes to preempt_disable() and task_struct, neither
of which is required. The above only needs some per-cpu storage in the
tracer implementation.