linux-kernel - Re: [PATCH v4 3/5] acpi_processor: do not mark present at boot but not onlined CPU as onlined

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Date:	Fri, 2 May 2014 13:32:19 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Toshi Kani <toshi.kani@...com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de,
	paul.gortmaker@...driver.com, JBeulich@...e.com, prarit@...hat.com,
	drjones@...hat.com, riel@...hat.com, gong.chen@...ux.intel.com,
	andi@...stfloor.org, lenb@...nel.org, rjw@...ysocki.net,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH v4 3/5] acpi_processor: do not mark present at boot but
 not onlined CPU as onlined

On Wed, 30 Apr 2014 15:25:51 -0600
Toshi Kani <toshi.kani@...com> wrote:

> On Mon, 2014-04-14 at 17:11 +0200, Igor Mammedov wrote:
> > acpi_processor_add() assumes that present at boot CPUs
> > are always onlined, it is not so if a CPU failed to become
> > onlined. As result acpi_processor_add() will mark such CPU
> > device as onlined in sysfs and following attempts to
> > online/offline it using /sys/device/system/cpu/cpuX/online
> > attribute will fail.
> > 
> > Do not poke into device internals in acpi_processor_add()
> > and touch "struct device { .offline }" attribute, since
> > for CPUs onlined at boot it's set by:
> >   topology_init() -> arch_register_cpu() -> register_cpu()
> > before ACPI device tree is parsed, and for hotplugged
> > CPUs it's set when userspace onlines CPU via sysfs.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> > ---
> > v2:
> >  - fix regression in v1 leading to NULL pointer dereference
> >    on CPU unplug, do not remove "pr->dev = dev;"
> > ---
> >  drivers/acpi/acpi_processor.c |    1 -
> >  1 files changed, 0 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> > index c29c2c3..42d66f8 100644
> > --- a/drivers/acpi/acpi_processor.c
> > +++ b/drivers/acpi/acpi_processor.c:q
> > @@ -404,7 +404,6 @@ static int acpi_processor_add(struct acpi_device *device,
> >  		goto err;
> >  
> >  	pr->dev = dev;
> > -	dev->offline = pr->flags.need_hotplug_init;
> 
> IIRC, this change was necessary to handle the case when maxcpus=X is
> specified at boot.  In this case, excessive CPU's dev->offline needs to
> be set to 1.  Can you verify this?
Option 'maxcpus' works just fine without and with this patch since a bit
earlier in acpi_processor_add() it exits in case of extra present CPUs:

#ifdef CONFIG_SMP
        if (pr->id >= setup_max_cpus && pr->id != 0)
                return 0;
#endif

and execution doesn't get to the point the patch touches.

The point is that acpi_processor_add() shouldn't touch
dev->offline at all and allow register_cpu() handle it.

> 
> Thanks,
> -Toshi
> 

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