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

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Date:	Mon, 14 Jan 2013 13:04:27 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	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 Mon, 14 Jan 2013, Linus Torvalds wrote:

> > - from view of driver, introducing async_synchronize_full() after
> > do_one_initcall() inside do_init_module() is like a sync probe
> > for drivers built as module, and cause this kind of deadlock easily.
> >
> > So could we revert the commit and fix the previous problems just
> > case by case? or other better fix?
> 
> There's no way in hell we take a "fix things one by one" approach.
> It's not going to work. And your suggestion seems to not do async
> discovery of devices in general, which is a *much* worse fix than
> anything else. It's just crazy.
> 
> But there are other approaches we might take. We might move the call to
> 
>     async_synchronize_full();
> 
> to other places. For example, maybe we're better off doing it at
> block/char device open instead?

How about skipping that call if the current thread is one of the async 
helpers?  Is it possible to detect when that happens?

Or maybe such a check should go inside async_synchronize_full() itself.

Alan Stern

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