Re: urbs are REFERENCE COUNTED!!! [was Re: [PATCH]generic usb serial driver]

On Mon, Apr 02, 2007 at 09:58:00AM -0400, Alan Stern wrote:
> On Mon, 2 Apr 2007, Greg KH wrote:
> 
> > On Mon, Apr 02, 2007 at 01:05:21AM -0700, David Brownell wrote:
> > > On Sunday 01 April 2007 8:16 pm, Greg KH wrote:
> > > > 
> > > > The way to properly solve this is to just never care about recycling
> > > > urbs.  Look at what the visor driver does as an example of how you can
> > > > just create a new urb for _every_ instance, fire it off, and then forget
> > > > about it.  It gets used and then automatically cleaned up.
> > > 
> > > That can certainly work, and it has the advantages of simplicity.
> > > I wouldn't use that strategy in every driver of course, but I'd
> > > guess it's fine for the serial and usblp cases mentioned here.
> > 
> > Why would you not want it in every driver?  As Oliver is finding out, a
> > lot of these "simple" drivers have races all due to their use of a
> > pre-allocated urb.  For what kind of situation would you feel this would
> > not work?
> 
> You don't need to do it in drivers that run in a single kernel thread, 
> just using the same URB over and over (like usb-storage).  Or in drivers 
> that simply resubmit the same URB from within the completion handler 
> (like khubd).
> 
> The difficulties arise because requests are generated from userspace 
> asynchronously with respect to completions.

Yes, which is usually the majority of the drivers we have :)

thanks,

greg k-h

-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
_______________________________________________
[email protected]
To unsubscribe, use the last form field at:
https://lists.sourceforge.net/lists/listinfo/linux-usb-devel