Re: device_remove_file and disconnect

[mat@mut38-1-82-67-62-65.fbx.proxad.net]

Hi,

On Wed, Jun 29, 2005 at 03:42:35PM -0700, Greg KH wrote:
> On Wed, Jun 29, 2005 at 10:17:59PM +0200, matthieu castet wrote:
> > Hi,
> > 
> > Greg KH wrote:
> > >On Wed, Jun 29, 2005 at 06:59:00PM +0200, matthieu castet wrote:
> > >
> > >>Hi,
> > >>
> > >>I have a question about sysfs interface.
> > >>
> > >>If you open a sysfs file created by a module, then remove it (rmmoding 
> > >>the module that create this sysfs file), then try to read the opened 
> > >>file, you often get strange result (segdefault or oppps).
> > >
> > >
> > >What file did you do this for?  The module count should be incremented
> > >if you do this, to prevent the module from being unloaded.
> > >
> > Ok, but if we unplug a device, then disconnect will be called even if we 
> > opened a sysfs file.
> 
> Yes but the device structure will still be in memory, so you will be ok.
> 
disconnect method isn't supposed to clear the device structure in memory ?

> > Couldn't be a race between the moment we read our private data and check 
> > it is valid and the moment we use it :
> > 
> > Process A (read/write sysfs file) 		Process B (disconnect)
> > recover our private data from struct device
> > check it is valid
> > 						free our private data
> > do operation on private data
> 
> No, you should not be freeing your private data on your own.  You should
> do that in the device release function.
But that's what I do !
I free it in  device release function = usb_disconnect in my case =
Process B. Process A is call by sysfs and don't seem serialized with
Process B.
Is there another place where I could free my private data later : I don't
think so.

The problem is that device_remove_file don't block if the sysfs file is
opened. So we must serialized A and B with mutex.
> 
> Again, any specific place in the kernel that you see not doing this?
I believe some drivers expected that sysfs read/write callback are always
called when the device is plugged so they don't check if
to_usb_interface/usb_get_intfdata return valid pointer.

Also I always see driver free their privatre data in device disconnect,
so if read/write from sysfs aren't serialized with device disconnect
there are still a possible race like I show in my example.



thanks,

Matthieu
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