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

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Date:	Tue, 15 Jan 2013 23:01:15 -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 Tue, 15 Jan 2013, Tejun Heo wrote:

> Hello, Arjan.
> 
> On Tue, Jan 15, 2013 at 04:25:54PM -0800, Arjan van de Ven wrote:
> > async fundamentally had the concept of a monotonic increasing number,
> > and that you could always wait for "everyone before me".
> > then people (like me) wanted exceptions to what "everyone" means ;-(
> > I'm ok with going back to a single space and simplify the world.
> 
> If we want (or need) finer grained operation, we'll probably have to
> head the other direction, so that we can definitively tell that an
> async operation belongs to domains system, module load A and B, so
> that each waiter knows what to wait for.
> 
> The current domain implementation is somewhere inbetween.  It's not
> completely simplistic system and at the same time not developed enough
> to do properly stacked flushing.

I like your idea of chronological synchronization: Insist that anybody
who wants to flush async jobs must get a cookie, and then only allow
them to wait for async jobs started after the cookie was issued.

I don't know if this is possible with the current implementation.  It 
would require changing every call to async_synchronize_*(), and in a 
nontrivial way.  But it might provide a proper solution to all these 
problems.

Can you think of any reasons why it wouldn't work in principle?  It 
would prevent code from doing "wait until all currently-running async 
jobs have finished" -- but arguably, nobody should be allowed to do 
that anyway.

Alan Stern

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