linux-kernel - Re: USB device cannot be reconnected and khubd "blocked for more than 120 seconds"

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Date:	Wed, 16 Jan 2013 12:51:27 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ming Lei <ming.lei@...onical.com>,
	Alex Riesen <raa.lkml@...il.com>, Jens Axboe <axboe@...nel.dk>,
	USB list <linux-usb@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: USB device cannot be reconnected and khubd "blocked for more
 than 120 seconds"

On Wed, 16 Jan 2013, Tejun Heo wrote:

> Hello, Alan.
> 
> On Wed, Jan 16, 2013 at 12:01:53PM -0500, Alan Stern wrote:
> > > The problem here is that "flush everything which comes before me" is
> > > used to order async jobs.  e.g. after async jobs probe the hardware
> > > they order themselves by flushing before registering them, so unless
> > 
> > I don't fully understand this example.  What is the point -- to make 
> > sure that asynchronously probed devices are registered in the order of 
> > their discovery?
> 
> People still want devices to be numbered to their physical ports and
> so on, so we keep the registeration order the same as natural
> (whatever that means) hardware order.
> 
> > If so, here's how to do it safely: Start up the async jobs in reverse
> > order of discovery.  Have each job acquire a cookie when it starts.  
> > Then each job needs to wait only for tasks that started after its
> > cookie was issued.
> 
> It's a bit clumsy but yeah I guess it could work.
> 
> > > There aren't too many which use async anyway so changing stuff
> > > shouldn't be too difficult but I think the simpicity or dumbness is
> > > one of major attractions of async, so it'd be nice to keep things that
> > > way and the PF_USED_ASYNC hack seems to be able to hold things
> > > together for now.
> > 
> > Nesting won't matter for the chronological approach.  I really think 
> > you should consider it more fully.  It's not a hack, and it doesn't 
> > need to be complicated.
> 
> There is benefit to the current dumb implementation in that drivers
> can use it without thinking too much, but yeah it could be that the
> flushing range limit isn't too much of restriction on top.  I don't
> know.  At this point, I'd prefer to remove request_module() from
> elevator init path for the problem at hand.  If we need something more
> involved, changing cookie usage rules definitely seems like an option.

A simpler approach might be to leave the existing synchronization 
mechanisms as they are, and use the chronological approach only for the 
case of loading a module (or wherever else someone wants to use it).

Alan Stern

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