Date: Tue, 27 Nov 2012 19:32:45 +0100
From: Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>
To: Toshi Kani <toshi.kani@hp.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>, Wen Congyang <wencongyang@gmail.com>,
        linux-acpi@vger.kernel.org, isimatu.yasuaki@jp.fujitsu.com,
        rjw@sisk.pl, lenb@kernel.org, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v3 3/3] acpi_memhotplug: Allow eject to proceed on
 rebind scenario
Message-ID: <20121127183245.GA4674@dhcp-192-168-178-175.profitbricks.localdomain>
References: <1353693037-21704-1-git-send-email-vasilis.liaskovitis@profitbricks.com>
 <1353693037-21704-4-git-send-email-vasilis.liaskovitis@profitbricks.com>
 <50B0F3DF.4000802@gmail.com>
 <20121126083634.GA4574@dhcp-192-168-178-175.profitbricks.localdomain>
 <50B3323E.7020907@cn.fujitsu.com>
 <1353975541.26955.182.camel@misato.fc.hp.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1353975541.26955.182.camel@misato.fc.hp.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2317
Lines: 59

On Mon, Nov 26, 2012 at 05:19:01PM -0700, Toshi Kani wrote:
> > >> Consider the following sequence of operations for a hotplugged memory
> > >> device:
> > >>
> > >> 1. echo "PNP0C80:XX" > /sys/bus/acpi/drivers/acpi_memhotplug/unbind
> > >> 2. echo 1 >/sys/bus/pci/devices/PNP0C80:XX/eject
> > >>
> > >> If we don't offline/remove the memory, we have no chance to do it in
> > >> step 2. After
> > >> step2, the memory is used by the kernel, but we have powered off it. It
> > >> is very
> > >> dangerous.
> > > 
> > > How does power-off happen after unbind? acpi_eject_store checks for existing
> > > driver before taking any action:
> > > 
> > > #ifndef FORCE_EJECT
> > > 	if (acpi_device->driver == NULL) {
> > > 		ret = -ENODEV;
> > > 		goto err;
> > > 	}
> > > #endif
> > > 
> > > FORCE_EJECT is not defined afaict, so the function returns without scheduling
> > > acpi_bus_hot_remove_device. Is there another code path that calls power-off?
> > 
> > Consider the following case:
> > 
> > We hotremove the memory device by SCI and unbind it from the driver at the same time:
> > 
> > CPUa                                                  CPUb
> > acpi_memory_device_notify()
> >                                        unbind it from the driver
> >     acpi_bus_hot_remove_device()
> 
> Can we make acpi_bus_remove() to fail if a given acpi_device is not
> bound with a driver?  If so, can we make the unbind operation to perform
> unbind only?

acpi_bus_remove_device could check if the driver is present, and return -ENODEV
if it's not present (dev->driver == NULL).

But there can still be a race between an eject and an unbind operation happening
simultaneously. This seems like a general problem to me i.e. not specific to an
acpi memory device. How do we ensure an eject does not race with a driver unbind
for other acpi devices?

Is there a per-device lock in acpi-core or device-core that can prevent this from
happening? Driver core does a device_lock(dev) on all operations, but this is
probably not grabbed on SCI-initiated acpi ejects.

thanks,

- Vasilis
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/