linux-kernel - Re: [RFC PATCH 0/3] sysfs: allow suicide

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Message-ID: <Pine.LNX.4.44L0.0903261005530.2945-100000@iolanthe.rowland.org>
Date:	Thu, 26 Mar 2009 10:21:23 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <htejun@...il.com>
cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alex Chiang <achiang@...com>, <greg@...ah.com>,
	<cornelia.huck@...ibm.com>, <kay.sievers@...y.org>,
	<rusty@...tcorp.com.au>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] sysfs: allow suicide

On Thu, 26 Mar 2009, Tejun Heo wrote:

> > The problem that I see is that we are missing support from the device
> > model for hotunplug.  Running the device remove method from process
> > context is required.  Typically hotplug controllers discover a
> > device has been removed or will be removed in interrupt context.
> > 
> > Therefore every hotplug driver I have looked at has it's own workqueue
> > to solve the problem of getting the notification of a hotplug event
> > from an inappropriate context.
> > 
> > So the general problem that I see is that I need a solution to trigger
> > a remove from interrupt context and that same solution will happen to
> > work just fine for sysfs.
> 
> I think the problem is more driver domain specific and not quite sure
> whether one size would fit all.  We have a lot of drivers in the tree.
> I think the best approach would be trying to move upwards from the
> bottom.  ie. Consolidate hotplug / error handling support from low
> level drivers to specific driver subsystem, from driver subsystems to
> higher layer (ie. block layer) and then see whether there can be more
> commonalities which can be factored, but the chance is that once
> things are pushed upwards enough, moving it into the kobject layer
> probably wouldn't worth the trouble.  Well, it's all speculations at
> this point tho.

It sounds like Eric's point is that sysfs_schedule_callback() is a
special-purpose interface devoted to sysfs, whereas a more generally
useful interface would allow delayed unregistration of any struct
device. (Or perhaps delayed invocation of an arbitrary function with a
struct device as the argument, but unregistration is the single most
important usage.)

The actual interface wouldn't be very different from 
sysfs_schedule_callback().  In fact, the only changes I see offhand are 
that the new routine would accept a pointer to struct device instead of 
a pointer to struct kobject and there wouldn't be any func argument.  

The idea is that this would come in useful both for suicidal sysfs 
attributes and for hot-unplug events detected by an interrupt handler.

But there's something I'm not clear on.  If hot-unplug events are
detected by an interrupt handler, then what about hot-plug events?  
Wouldn't they be detected by the same interrupt handler?  Obviously you
can't register new devices in interrupt context, so there must be a
workqueue or kernel thread involved somewhere.  Shouldn't the two types
of events be managed by the same workqueue/thread?

Alan Stern

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