linux-kernel - Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able

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Message-ID: <alpine.DEB.2.20.1710051808280.2083@nanos>
Date:   Thu, 5 Oct 2017 18:16:23 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Julia Cartwright <julia@...com>
cc:     Steven Rostedt <rostedt@...dmis.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        bigeasy@...utronix.de, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Dean Luick <dean.luick@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        linux-rdma@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>,
        Sebastian Sanchez <sebastian.sanchez@...el.com>
Subject: Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able_nort()

On Thu, 5 Oct 2017, Julia Cartwright wrote:

> On Thu, Oct 05, 2017 at 11:55:39AM -0400, Steven Rostedt wrote:
> > On Thu, 5 Oct 2017 10:37:59 -0500
> > Julia Cartwright <julia@...com> wrote:
> > 
> > > On Thu, Oct 05, 2017 at 05:27:30PM +0200, Thomas Gleixner wrote:
> > > > On Thu, 5 Oct 2017, Julia Cartwright wrote:  
> > > > > On Tue, Oct 03, 2017 at 12:49:19PM -0300, Arnaldo Carvalho de Melo wrote:  
> > > > > > -	preempt_disable();
> > > > > > +	preempt_disable_nort();
> > > > > >  	this_cpu_inc(*sc->buffers_allocated);  
> > > > > 
> > > > > Have you tried this on RT w/ CONFIG_DEBUG_PREEMPT?  I believe that the
> > > > > this_cpu_* operations perform a preemption check, which we'd trip.  
> > > > 
> > > > Good point. Changing this to migrate_disable() would do the trick.  
> > > 
> > > Wouldn't we still trip the preempt check even with migration disabled?
> > > In another thread I asked the same question: should the preemption
> > > checks here be converted to migration-checks in RT?
> > 
> > Is it a "preemption check"?
> 
> Sorry if I was unclear, more precisely: the this_cpu_* family of
> accessors, w/ CONFIG_DEBUG_PREEMPT currently spits out a warning when
> the caller is invoked in a context where preemption is enabled.
> 
> The check is shared w/ the smp_processor_id() check, as implemented in
> lib/smp_processor_id.c.  It effectively boils down to a check of
> preempt_count() and irqs_disabled().

Except that on RT that check cares about the migrate disable state. You can
invoke this_cpu_* and smp_processor_id() in preemptible/interruptible
context because of:

	if (cpumask_equal(current->cpus_ptr, cpumask_of(this_cpu)))
		goto out;

That's true even on mainline.

But you are right that this check needs some update because
migrate_disable() does not immediately update the allowed cpumask IIRC.

Thanks,

	tglx