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

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Message-ID: <20110809113743.GA32299@redhat.com>
Date:	Tue, 9 Aug 2011 14:37:43 +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 Mon, Aug 08, 2011 at 07:19:31PM +0200, Hans J. Koch wrote:
> On Mon, Aug 08, 2011 at 09:24:31AM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> Should I hack up a patch to remove the lock, or do you have anything in your
> pipeline?


Please do.

> > > 
> > > > 
> > > > > 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
> 
> Hope that works at all times...
> Anyway, the spin_lock in uio_pci_generic.c definetly doesn't help.
> 
> Thanks,
> Hans

True.
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