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 10:32:04 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Alex Riesen <raa.lkml@...il.com>,
	Alan Stern <stern@...land.harvard.edu>,
	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"

Hello, Linus.

On Tue, Jan 15, 2013 at 09:36:57AM -0800, Linus Torvalds wrote:
> Tejun, comments? You can see the whole thread on lkml, but the basic
> problem is that the module loading doing the unconditional
> async_synchronize_full() has caused problems, because we have
> 
>  - load module A
>    - module A does per-controller async discovery of its devices (eg
> scsi or ata probing)
>    - in the async thread, it initializes somethign that needs another
> module B (in this case the default IO scheduler module)
>       - modprobe for B loads the IO scheduler module successfully
>           at the end of the module load, it does
> async_synchronize_full() to make sure load_module won't return before
> the module is ready
>           *DEADLOCK*, because the async_synchronize_full() thing
> actually waits for not the module B async code (it didn't have any),
> but for the module *A* async code, which is waiting for module B to
> finish.

I think the root problem here, apart from request_module() from block
- which is a bit nasty but making that part completely async would too
be quite nasty albeit in a different way - is that
async_synchronize_full() is way too indescriminate.  It's something
only suitable for things like the end of system init.

I'm wondering whether what we need is a rudimentray nesting like the
following.

finished_loading()
{
	blah blah;

	cookie = async_current_cookie();

	do init calls;

	async_synchronize_upto(cookie);

	blah blah;
}

The nesting here would be an approximation as the dependency recorded
here is chronological.  I *suspect* this should be safe unless the
module is doing something weird.  Need to think more about it.  One
way or the other, I think what we need is some form of scoping for
flushing async ops.

BTW, the current synchronization is broken - cookie isn't transferred
to running->domain in queueing order but __lowest_in_progress()
assumes that.  I think I broke that while converting it to workqueue.

Anyways, working on it.

Thanks.

-- 
tejun
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