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

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Message-ID: <20091007192507.2784266a@lxorguk.ukuu.org.uk>
Date:	Wed, 7 Oct 2009 19:25:07 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oliver@...kum.org>, 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

> > It would probably be cleaner if they could lock against each other
> 
> What you mean isn't clear.  After all, open sometimes has to call
> resume.  So how could resume lock against open?

Probably it needs a counting lock as the code is currently structured -
which is a bit ugly. What paths do we end up going through the device
open method into resume in the same thread ?

> Does this imply that unthrottle should try to autoresume?  There does 
> appear to be a potential race between unthrottle and autosuspend.

The more I look at it the more it implies to me that the ldiscs doing
this should instead be taught some better manners instead. The real nasty
is that a driver might not have initialised the locking it needs do that
exclusion until open occurs. I think n_tty is probably the only offender
and if so I'd rather fix that and make it a rule that you don't do that,
trying to fix it other ways is going to be more horrible I imagine.
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