linux-kernel - Re: [PATCH] uio/gen-pci: don't enable interrupts in ISR

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Date:	Tue, 9 Aug 2011 15:02:07 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Chris Wright <chrisw@...hat.com>,
	"Hans J. Koch" <hjk@...sjkoch.de>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Anthony Foiani <anthony.foiani@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uio/gen-pci: don't enable interrupts in ISR

On Tue, Aug 09, 2011 at 01:43:46PM +0200, Sebastian Andrzej Siewior wrote:
> Michael S. Tsirkin wrote:
> >>It seems to me that you could drop this "user block" thing
> >>since you never change anything outside of this command register and it
> >>does not stop the race.
> >
> >I don't think so: if we did, we would lose userspace modifications to
> >other bits such as io enable, and there's no way to guess what their
> >values should be.
> 
> How so?
> 
> Sebastian

Let's assume we start with e.g. io enable bit cleared.


    user                                kernel
 read config                               |
 set io enable                             |
     |                                  interrupt                                                
     |                              read + set interrupt mask
 write config back                         |
					write



We end up with io enable bit cleared. Locking around rmw
that we have fixes this race.

-- 
MST
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