linux-kernel - Re: [PATCH, RFC] sysfs: only allow one scheduled removal callback per kobj

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Date:	Wed, 11 Mar 2009 20:22:28 -0700
From:	Greg KH <gregkh@...e.de>
To:	Alex Chiang <achiang@...com>, Tejun Heo <tj@...nel.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>, jbarnes@...tuousgeek.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] sysfs: only allow one scheduled removal callback
	per kobj

On Wed, Mar 11, 2009 at 06:27:37PM -0600, Alex Chiang wrote:
> * Tejun Heo <tj@...nel.org>:
> > Alex Chiang wrote:
> > > * Greg KH <gregkh@...e.de>:
> > >> On Tue, Mar 10, 2009 at 05:20:27PM -0600, Alex Chiang wrote:
> > >>> Hi Vegard, sysfs folks,
> > >>>
> > >>> Vegard was nice enough to test my PCI remove/rescan patches under
> > >>> kmemcheck. Maybe "torture" is a more appropriate term. ;)
> > >>>
> > >>> My patch series introduces a sysfs "remove" attribute for PCI
> > >>> devices, which will remove that device (and child devices).
> > >>>
> > >>> 	http://thread.gmane.org/gmane.linux.kernel.pci/3495
> > >>>
> > >>> Vegard decided that he wanted to do something like:
> > >>>
> > >>> 	# while true ; do echo 1 > /sys/bus/pci/devices/.../remove ; done
> > >>>
> > >>> which caused a nasty oops in my code. You can see the results of
> > >>> his testing in the thread I referenced above.
> > >>>
> > >>> After looking at my code for a bit, I decided that maybe it
> > >>> wasn't completely my fault. ;) See, I'm using device_schedule_callback()
> > >> why?  Are you really in interrupt context here to need to do the remove
> > >> at a later time?
> > > 
> > > What other interface can I use to remove objects from sysfs?
> > 
> > I haven't read your code yet but I seem to recall doing something
> > similar.  Ah.. okay, this one didn't get in and I forgot about this.
> > 
> >   http://thread.gmane.org/gmane.linux.kernel/582130
> > 
> > But, yeah, committing suicide is currently quite hariy.  I tought SCSI
> > did it correctly with all the grab/release dances.  Does SCSI have the
> > problem too?
> 
> I haven't dived into the SCSI code yet, but they are doing some
> sort of magic that I don't understand with their state machine.
> 
> Regardless, I think we have two issues.
> 
> 	1. The existing callback mechanism that everyone hates
> 	has a "bug".
> 
> 	2. Your suicide patches haven't made it into mainline yet.
> 
> The reason that I think that the "bug" is with the callback
> mechanism is because any caller can repeatedly schedule suicide
> over and over again, and the callback handler will eventually get
> a stale pointer. Rather than make all the callsites handle the
> locking, doesn't it make more sense for the infrastructure to do
> it?
> 
> I realize we're trying to fix something that everyone wants to go
> away, but the PCI rescan patches add some pretty useful
> functionality and pretty much ready to go except for this. I
> could add the bookkeeping into my suicide path, but that's
> actually a slightly bigger patch, because now I have to malloc my
> own callback structs. And again, I think it's more appropriate to
> put that sort of code into the core.
> 
> Can we fix 1 in the short term and move towards 2 as the real
> solution?

I have no objection to this plan.

thanks,

greg k-h
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