Subject: Re: [PATCH 4/4] usb: libusual: locking cleanup
From: Daniel Walker <dwalker@mvista.com>
To: Pete Zaitcev <zaitcev@redhat.com>
Cc: akpm@linux-foundation.org, mingo@elte.hu, linux-kernel@vger.kernel.org,
       linux@bohmer.net, jonathan@jonmasters.org, matthias.kaehlcke@gmail.com,
       kjwinchester@gmail.com
In-Reply-To: <20071224061220.ceed2bf8.zaitcev@redhat.com>
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Date: Mon, 24 Dec 2007 08:04:08 -0800
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On Mon, 2007-12-24 at 06:12 -0800, Pete Zaitcev wrote:
> On Sun, 23 Dec 2007 08:46:37 -0800, Daniel Walker <dwalker@mvista.com> wrote:
> 
> > I noticed you also have a spinlock held in usu_probe_thread(), the
> > usu_lock.. That spinlock would preclude anything inside request_module()
> > from sleeping..
> 
> The usu_lock is not held across request_module. In fact, it can be
> easily taken from inside request_module, when it invokes modprobe.
> Stop scaring me :-)

Your right, it's just outside .. I still don't think it could deadlock,
since I don't see a code path to recursively get back into those
libusual functions..

> > One thing that has bothered me is that I don't see a reason why this
> > couldn't become a complete, yet you have a comment which says that it
> > can't be a complete.. I honestly didn't understand the comment.. I would
> > imagine that you tried a complete , and it didn't work?
> 
> Yes, it was a completition initially. But suppose you have two storage
> devices, plugged in across a reboot. Two threads are created and have to
> wait until the libusual's init function ends. Since we post one
> completion,
> only one thread continues.

Ok .. The mutex should just prevent them from running at the same time,
but they should both run eventually.

Daniel

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