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 09:30:18 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	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"

On Sun, Jan 13, 2013 at 11:15 PM, Ming Lei <ming.lei@...onical.com> wrote:
>
> The deadlock problem is caused by calling request_module() inside
> async function of do_scan_async(), and it was introduced by Linus's
> below commit:
>
> commit d6de2c80e9d758d2e36c21699117db6178c0f517
> Author: Linus Torvalds <torvalds@...ux-foundation.org>
> Date:   Fri Apr 10 12:17:41 2009 -0700
>
>     async: Fix module loading async-work regression
>
> IMO, maybe the commit isn't a proper fix, considered the
> below fact:
>
> - it isn't good to allow async function to be marked as __init

Immaterial. For modules, __init is a non-issue. For non-modules, the
synchronization elsewhere is sufficient.

> - any user mode shouldn't expect that the device is ready just
> after completing of 'insmod'

Bullshit. That expectation is just a fact. People insmod a device
driver, and mount the device immediately in scripts.

We do not say "user mode shouldn't". Seriously. EVER. User mode
*does*, and we deal with it. Learn it now, and stop ever saying that
again.

This is really starting to annoy me. Kernel developers who say "user
mode should be fixes to not do that" should go somewhere else. The
whole and *only* point of a kernel is to hide these kinds of issues
from user mode, and make things "just work" in user mode. User mode
should not ever worry about "oh, doing X can trigger a module load, so
now the device might not be available immediately, so I should delay
and re-try until it is".

That's just f*cking crazy talk.

We have a very simple rule in the kernel: we don't break user space. EVER.

Learn that rule. I don't ever want to hear "any user mode shouldn't
expect" again. User mode *does* expect. End of discussion.

> - 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?

              Linus
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