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

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Date:	Thu, 8 Oct 2009 12:58:08 -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 Donnerstag, 8. Oktober 2009 16:58:39 schrieb Alan Stern:
> > On Thu, 8 Oct 2009, Oliver Neukum wrote:
> > > Am Mittwoch, 7. Oktober 2009 23:34:12 schrieb Alan Stern:
> 
> > > > 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.
> 
> We have to submit URBs under that lock.

Yes.  What's your point?

> > > 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.
> 
> We have at least three reasons URBs should not be submitted.
> - closure
> - throttling
> - suspension
> Resume() should not submit if either closure or throttling are active,
> neither should unthrottle() resubmit if closure or suspension are active.

True.  Nor should open() submit if throttling is active.  Feel free to 
use three separate flags.  :-)

Alan Stern

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