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

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Message-ID: <CA+55aFzm4k2CyQ_mZVHyeasjw4ybqfO2SpJrZZNZeUsSW8PB=g@mail.gmail.com>
Date:	Tue, 15 Jan 2013 16:36:34 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.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>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: USB device cannot be reconnected and khubd "blocked for more than
 120 seconds"

On Tue, Jan 15, 2013 at 3:50 PM, Tejun Heo <tj@...nel.org> wrote:
>
> For now, I'm gonna implement simple "I'm not gonna wait for myself"
> self-deadlock avoidance.

You can't really do that. Or rather, it won't *help*.

The thing is, the module loading in particular is not necessarily
happening in the same context as what *started* the module loading. A
module loader will request the module from user space, and then later
user space - through possibly a totally unrelated process - will
finish it. So there is no "myself". There's not even necessarily any
relationship that the kernel even knows about, because the module
loading request can have gone from usermode_helper over something like
dbus to systemd.

See?

There's a reason I asked for a warning for this. Or the "let's flag
the current thread if it ever started anything asynchronous". Because
it's complicated.

          Linus
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