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 16:40:36 -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 4:36 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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.

Btw, the sequence counter (that is *not* taking anything else into
account) is good enough in practice, exactly because the common case
for module loading is actually that nothing in the module init
sequence is done asynchronously.

Yes, device discovery (particularly for block devices) is often
asynchronous. But the modules it then asks to load usually wouldn't
be. So if we just have the flag "did this thread ever even start async
work" over the module init sequence, we can just avoid the async
serialization entirely for that case, and it breaks the deadlock chain
nicely in practice.

Only of a block device does async work and then wants to load another
module that does more async work in its init routine would it then
break. But at that point, I'll happily just put my foot down and tell
people they are crazy, and "Let's not do that kind of crap".

            Linus
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