Re: MSI interrupts and disable_irq

From: Stephen Hemminger
Date: Fri Sep 28 2007 - 23:08:34 EST

On Fri, 28 Sep 2007 22:47:16 -0400
Jeff Garzik <jgarzik@xxxxxxxxx> wrote:

> Ayaz Abdulla wrote:
> > I am trying to track down a forcedeth driver issue described by bug 9047
> > in bugzilla (2.6.23-rc7-git1 forcedeth w/ MCP55 oops under heavy load).
> > I added a patch to synchronize the timer handlers so that one handler
> > doesn't accidently enable the IRQ while another timer handler is running
> > (see attachment 'Add timer lock' in bug report) and for other processing
> > protection.
> >
> > However, the system still had an Oops. So I added a lock around the
> > nv_rx_process_optimized() and the Oops has not happened (see attachment
> > 'New patch for locking' in bug report). This would imply a
> > synchronization issue. However, the only callers of that function are
> > the IRQ handler and the timer handlers (in non-NAPI case). The timer
> > handlers use disable_irq so that the IRQ handler does not contend with
> > them. It looks as if disable_irq is not working properly.
> >
> > This issue repros only with MSI interrupt and not legacy INTx
> > interrupts. Any ideas?
> (added linux-kernel to CC, since I think it's more of a general kernel
> issue)
> To be brutally frank, I always thought this disable_irq() mess was a
> hack both ugly and fragile. This disable_irq() work that appeared in a
> couple net drivers was correct at the time, so I didn't feel I had the
> justification to reject it, but it still gave me a bad feeling.
> I think the scenario you outline is an illustration of the approach's
> fragility: disable_irq() is a heavy hammer that originated with INTx,
> and it relies on a chip-specific disable method (kernel/irq/manage.c)
> that practically guarantees behavior will vary across MSI/INTx/etc.
> Practices like forcedeth's unique locking work for a time, but it should
> be a warning sign any time you stray from the normal spin_lock_irqsave()
> method of synchronization.
> Based on your report, it is certainly possible that there is a problem
> with MSI's desc->chip->disable() method... but I would actually
> recommend working around the problem by making the forcedeth locking
> more standardized by removing all those disable_irq() hacks.
> Using spinlocks like other net drivers (note: avoid NETIF_F_LLTX
> drivers) has a high probability of both fixing your current problem, and
> giving forcedeth a more stable foundation for the long term. In my
> humble opinion :)

I'll try and clean it up if the author doesn't get to it first.

Stephen Hemminger <shemminger@xxxxxxxxxxxxxxxxxxxx>

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