linux-kernel - Re: [PATCH 4/5] usb_serial: Kill port mutex

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Date:	Thu, 8 Oct 2009 10:58:39 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 4/5] usb_serial: Kill port mutex

On Thu, 8 Oct 2009, Oliver Neukum wrote:

> Am Mittwoch, 7. Oktober 2009 23:34:12 schrieb Alan Stern:
> > I'm losing track of the original point of this thread.  IIRC, the
> > problem is how the resume method should know whether or not to submit
> > the receive URB(s).  It can't afford to acquire the port mutex because
> > it might be called by open or close, at which time the mutex is already
> > held.
> >
> > Other schemes could work, but to me it seems simplest to rely on a flag
> > protected by a spinlock.  The flag would mean "URBs are supposed to be
> > queued unless we are suspended".  It would be set by open and
> > unthrottle, and cleared by close and throttle.
> 
> 1. Why a spinlock?

Because the amount of work involved seems too small for a mutex.  But 
you could use a mutex if you wanted, since everything occurs in process 
context.

> 2. Can we get by with only one flag?

If all you want to do is answer a single question ("Should URBs be 
submitted") then a single flag should be all you need.  Why, do you 
think more information will be necessary?  You can always add more.

Alan Stern

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