Date:	Mon, 09 Jul 2012 15:15:44 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	linux-acpi@...r.kernel.org, lenb@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] acpi : cpu hot-remove returns error number when
 cpu_down() fails

On Mon, 2012-07-09 at 16:55 +0530, Srivatsa S. Bhat wrote:
> On 07/09/2012 08:01 AM, Yasuaki Ishimatsu wrote:
> > Hi Srivatsa,
> > 
> > Thank you for your reviewing.
> > 
> > 2012/07/06 18:51, Srivatsa S. Bhat wrote:
> >> On 07/06/2012 08:46 AM, Yasuaki Ishimatsu wrote:
> >>> Even if cpu_down() fails, acpi_processor_remove() continues to remove the cpu.
> >>
> >> Ouch!
> >>
> >>> But in this case, it should return error number since some process may run on
> >>> the cpu. If the cpu has a running process and the cpu is turned the power off,
> >>> the system cannot work well.
> >>>
> >>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
> >>>
> >>> ---
> >>>   drivers/acpi/processor_driver.c |   18 ++++++++++++------
> >>>   1 file changed, 12 insertions(+), 6 deletions(-)
> >>>
> >>> Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
> >>> ===================================================================
> >>> --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c	2012-06-25 04:53:04.000000000 +0900
> >>> +++ linux-3.5-rc4/drivers/acpi/processor_driver.c	2012-07-05 21:02:58.711285382 +0900
> >>> @@ -610,7 +610,7 @@ err_free_pr:
> >>>   static int acpi_processor_remove(struct acpi_device *device, int type)
> >>>   {
> >>>   	struct acpi_processor *pr = NULL;
> >>> -
> >>> +	int ret;
> >>>
> >>>   	if (!device || !acpi_driver_data(device))
> >>>   		return -EINVAL;
> >>> @@ -621,8 +621,9 @@ static int acpi_processor_remove(struct
> >>>   		goto free;
> >>>
> >>>   	if (type == ACPI_BUS_REMOVAL_EJECT) {
> >>> -		if (acpi_processor_handle_eject(pr))
> >>> -			return -EINVAL;
> >>> +		ret = acpi_processor_handle_eject(pr);
> >>> +		if (ret)
> >>> +			return ret;
> >>>   	}
> >>>
> >>>   	acpi_processor_power_exit(pr, device);
> >>> @@ -841,12 +842,17 @@ static acpi_status acpi_processor_hotadd
> >>>
> >>>   static int acpi_processor_handle_eject(struct acpi_processor *pr)
> >>>   {
> >>> -	if (cpu_online(pr->id))
> >>> -		cpu_down(pr->id);
> >>> +	int ret;
> >>> +
> >>> +	if (cpu_online(pr->id)) {
> >>> +		ret = cpu_down(pr->id);
> >>> +		if (ret)
> >>> +			return ret;
> >>> +	}
> >>>
> >>
> >> Strictly speaking, this is not thorough enough. What prevents someone
> >> from onlining that same cpu again, at this point?
> >> So, IMHO, you need to wrap the contents of this function inside a
> >> get_online_cpus()/put_online_cpus() block, to prevent anyone else
> >> from messing with CPU hotplug at the same time.
> > 
> > If I understand your comment by mistake, please let me know.
> > If the contents is wrapped a inside get_online_cpus()/put_online_cpus() block
> > as below, cpu_down() will stop since cpu_down() calls cpu_hotplug_begin() and
> > cpu_hotplug_begin() waits for cpu_hotplug.refcount to become 0.
> > 
> 
> You are right. Sorry, I overlooked that.
> 
> > +	get_online_cpus()
> > +	if (cpu_online(pr->id)) {
> > +		ret = cpu_down(pr->id);
> > +		if (ret)
> > +			return ret;
> > +	}
> > +	put_online_cpus()
> > 
> > I think following patch can prevent it correctly. How about the patch?
> >
> > ---
> >  drivers/acpi/processor_driver.c |   12 ++++++++++++
> >  kernel/cpu.c                    |    8 +++++---
> >  2 files changed, 17 insertions(+), 3 deletions(-)
> > 
> > Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
> > ===================================================================
> > --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c	2012-07-09 09:59:01.280211202 +0900
> > +++ linux-3.5-rc4/drivers/acpi/processor_driver.c	2012-07-09 11:05:34.559859236 +0900
> > @@ -844,14 +844,26 @@ static int acpi_processor_handle_eject(s
> >  {
> >  	int ret;
> > 
> > +retry:
> >  	if (cpu_online(pr->id)) {
> >  		ret = cpu_down(pr->id);
> >  		if (ret)
> >  			return ret;
> >  	}
> > 
> > +	get_online_cpus();
> > +	/*
> > +	 * Someone might online the cpu again at this point. So we check that
> > +	 * cpu has been onlined or not. If cpu is online, we try to offline
> > +	 * the cpu again.
> > +	 */
> > +	if (cpu_online(pr->id)) {
> 
> How about this:
> 	if (unlikely(cpu_online(pr->id)) {
> since the probability of this happening is quite small...
> 
> > +		put_online_cpus();
> > +		goto retry;
> > +	}
> >  	arch_unregister_cpu(pr->id);
> >  	acpi_unmap_lsapic(pr->id);
> > +	put_online_cpus();
> >  	return ret;
> >  }
> 
> This retry logic doesn't look elegant, but I don't see any better method :-(

Another possible option is to fail the request instead of retrying it.

It would be quite challenging to allow on-lining and off-lining
operations to run concurrently.  In fact, even if we close this window,
there is yet another window right after the new put_online_cpus() call.
This CPU may become online before calling _EJ0 in the case of
hot-remove.

This goes beyond the scope of this patch, but IMHO, we should serialize
in the request level.  That is, a new on-lining request should not be
allowed to proceed until the current request is complete.  This scheme
only allows a single operation at a time per OS instance, but I do not
think it is a big issue.

Serializing in the request level is also important when supporting
container hot-remove, which can remove multiple children objects under a
parent container object.  For instance, a Node hot-remove may also
remove multiple processors underneath of it.  This kind of the
operations has to make sure that all children objects are remained
off-line until it ejects the parent object.  

Thanks,
-Toshi


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