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Date:   Thu, 28 Nov 2019 16:56:12 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 12/14] torture: Replace cpu_up/down with
 device_online/offline
Message-ID: <20191128165611.7lmjaszjl4gbo7u2@e107158-lin.cambridge.arm.com>
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 <20191127214725.GG2889@paulmck-ThinkPad-P72>
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On 11/27/19 13:47, Paul E. McKenney wrote:
> On Mon, Nov 25, 2019 at 11:27:52AM +0000, Qais Yousef wrote:
> > The core device API performs extra housekeeping bits that are missing
> > from directly calling cpu_up/down.
> > 
> > See commit a6717c01ddc2 ("powerpc/rtas: use device model APIs and
> > serialization during LPM") for an example description of what might go
> > wrong.
> > 
> > This also prepares to make cpu_up/down a private interface for anything
> > but the cpu subsystem.
> > 
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> > CC: Davidlohr Bueso <dave@stgolabs.net>
> > CC: "Paul E. McKenney" <paulmck@kernel.org>
> > CC: Josh Triplett <josh@joshtriplett.org>
> > CC: linux-kernel@vger.kernel.org
> 
> Looks fine from an rcutorture viewpoint, but why not provide an API
> that pulled lock_device_hotplug() and unlock_device_hotplug() into the
> online/offline calls?

I *think* the right way to do what you say is by doing lock_device_hotplug()
inside device_{online, offline}() - which affects all drivers not just the CPU.

And even then, I think we need to refcount it so nested calls won't deadlock.

I don't know if this can break any rule or not. If Greg thinks it's okay I'd be
happy to post some patches that do just that.

Thanks

--
Qais Yousef

> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/torture.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/torture.c b/kernel/torture.c
> > index 7c13f5558b71..12115024feb2 100644
> > --- a/kernel/torture.c
> > +++ b/kernel/torture.c
> > @@ -97,7 +97,9 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
> >  			 torture_type, cpu);
> >  	starttime = jiffies;
> >  	(*n_offl_attempts)++;
> > -	ret = cpu_down(cpu);
> > +	lock_device_hotplug();
> > +	ret = device_offline(get_cpu_device(cpu));
> > +	unlock_device_hotplug();
> >  	if (ret) {
> >  		if (verbose)
> >  			pr_alert("%s" TORTURE_FLAG
> > @@ -148,7 +150,9 @@ bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
> >  			 torture_type, cpu);
> >  	starttime = jiffies;
> >  	(*n_onl_attempts)++;
> > -	ret = cpu_up(cpu);
> > +	lock_device_hotplug();
> > +	ret = device_online(get_cpu_device(cpu));
> > +	unlock_device_hotplug();
> >  	if (ret) {
> >  		if (verbose)
> >  			pr_alert("%s" TORTURE_FLAG
> > @@ -192,17 +196,20 @@ torture_onoff(void *arg)
> >  	for_each_online_cpu(cpu)
> >  		maxcpu = cpu;
> >  	WARN_ON(maxcpu < 0);
> > -	if (!IS_MODULE(CONFIG_TORTURE_TEST))
> > +	if (!IS_MODULE(CONFIG_TORTURE_TEST)) {
> > +		lock_device_hotplug();
> >  		for_each_possible_cpu(cpu) {
> >  			if (cpu_online(cpu))
> >  				continue;
> > -			ret = cpu_up(cpu);
> > +			ret = device_online(get_cpu_device(cpu));
> >  			if (ret && verbose) {
> >  				pr_alert("%s" TORTURE_FLAG
> >  					 "%s: Initial online %d: errno %d\n",
> >  					 __func__, torture_type, cpu, ret);
> >  			}
> >  		}
> > +		unlock_device_hotplug();
> > +	}
> >  
> >  	if (maxcpu == 0) {
> >  		VERBOSE_TOROUT_STRING("Only one CPU, so CPU-hotplug testing is disabled");
> > -- 
> > 2.17.1
> >