linux-kernel - Re: refcounting drivers' data structures used in sysfs buffers

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Date:	Fri, 9 Mar 2007 11:32:27 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oneukum@...e.de>
cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Maneesh Soni <maneesh@...ibm.com>, <gregkh@...e.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: refcounting drivers' data structures used in sysfs buffers

On Fri, 9 Mar 2007, Oliver Neukum wrote:

> Am Donnerstag, 8. März 2007 17:02 schrieb Alan Stern:
> > On Thu, 8 Mar 2007, Oliver Neukum wrote:
> > 
> > > Hi,
> > > 
> > > after a lightning bolt from high above I've been looking into refcounting
> > > the data structures drivers use to provide the data used to refill sysfs
> > > buffers. I've come to the following conclusion.
> > > 
> > > 1. struct sysfs_buffer must have a struct kref * and probably a destructor
> > > pointer
> > > 2. drivers must be able to pass these pointers through an extended
> > > device_create_file()
> > > 3. Drivers must use refcounting if they want to use attributes
> > > 4. read/write/poll must do refcounting
> > > 
> > > I am not sure where to store the pointers. struct sysfs_dirent() looks
> > > like the obvious choice. Comments?
> > 
> > Can you explain the reasoning that led to these conclusions?  And what 
> > exactly was your lightning bolt?
> 
> The old race between disconnect and IO to attribute via sysfs again.
> If I cannot disassociate the drivers from the buffers in the buffers, drivers
> must not deallocate the data necessary to answer sysfs callbacks while
> a buffer exists.

Why wouldn't you be able to dissociate a driver from a buffer?  That was 
the whole point of adding .orphan to sysfs_buffer and creating 
sysfs_buffer_collection -- it was supposed to solve exactly this race.

Alan Stern

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