linux-kernel - Re: [PATCH 1/4] Driver core: Add offline/online device operations

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Date:	Thu, 2 May 2013 06:57:28 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Toshi Kani <toshi.kani@...com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	isimatu.yasuaki@...fujitsu.com,
	vasilis.liaskovitis@...fitbricks.com, Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 1/4] Driver core: Add offline/online device operations

On Thu, May 02, 2013 at 02:27:30PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> In some cases, graceful hot-removal of devices is not possible,
> although in principle the devices in question support hotplug.
> For example, that may happen for the last CPU in the system or
> for memory modules holding kernel memory.
> 
> In those cases it is nice to be able to check if the given device
> can be gracefully hot-removed before triggering a removal procedure
> that cannot be aborted or reversed.  Unfortunately, however, the
> kernel currently doesn't provide any support for that.
> 
> To address that deficiency, introduce support for offline and
> online operations that can be performed on devices, respectively,
> before a hot-removal and in case when it is necessary (or convenient)
> to put a device back online after a successful offline (that has not
> been followed by removal).  The idea is that the offline will fail
> whenever the given device cannot be gracefully removed from the
> system and it will not be allowed to use the device after a
> successful offline (until a subsequent online) in analogy with the
> existing CPU offline/online mechanism.
> 
> For now, the offline and online operations are introduced at the
> bus type level, as that should be sufficient for the most urgent use
> cases (CPUs and memory modules).  In the future, however, the
> approach may be extended to cover some more complicated device
> offline/online scenarios involving device drivers etc.
> 
> The lock_device_hotplug() and unlock_device_hotplug() functions are
> introduced because subsequent patches need to put larger pieces of
> code under device_hotplug_lock to prevent race conditions between
> device offline and removal from happening.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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