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, 02 May 2014 11:23:43 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Igor Mammedov <imammedo@...hat.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 Fri, 2014-05-02 at 13:32 +0200, Igor Mammedov wrote:
> 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.

This is a separate topic, but I feel that the above code should not be
necessary...

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

Sorry, I had confused with cpu->dev.offline in register_cpu() in my
recollection.  Yes, I agree that we should let register_cpu() to handle
it.

Acked-by: Toshi Kani <toshi.kani@...com>

Thanks,
-Toshi





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