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

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Date:	Mon, 8 Aug 2011 09:24:31 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Hans J. Koch" <hjk@...sjkoch.de>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Wright <chrisw@...hat.com>,
	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 Fri, Aug 05, 2011 at 02:15:07AM +0200, Hans J. Koch wrote:
> On Fri, Aug 05, 2011 at 12:04:13AM +0300, Michael S. Tsirkin wrote:
> > On Thu, Aug 04, 2011 at 10:46:06PM +0200, Sebastian Andrzej Siewior wrote:
> > > As reported by Anthony in a short way:
> > > 
> > > |irq 17 handler uio_interrupt+0x0/0x68 enabled interrupts
> > > |NIP [c0069d84] handle_irq_event_percpu+0x260/0x26c
> > > 
> > > The problem here is that spin_unlock_irq() enables the interrupts which
> > > is a no-no in interrupt context because they always run with interrupts
> > > disabled. This is the case even if IRQF_DISABLED has not been specified
> > > since v2.6.35. Therefore this patch uses simple spin_locks().
> > > 
> > > Looking at it further here is only one spot where the lock is hold. So
> > > giving the fact that an ISR is not reentrant and is not executed on two
> > > cpus at the same time why do we need a lock here?
> > 
> > I'm not sure anymore. I think the idea was to use
> > it for synchronization down the road somehow,
> > but it never materialized. Let's drop that lock completely.
> 
> That sounds reasonable.
> 
> > 
> > > The driver lacks of ->irqcontrol function so I guess the interrupt is
> > > enabled via direct PCI-access in userland.
> > 
> > Through sysfs.
> 
> How? With /sys/devices/pci.../enable ?
> 
> Thanks,
> Hans

No. By writing to the command register using
/sys/bus/pci/devices/.../config

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