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    SubjectRe: MSI interrupts and disable_irq
    On Fri, 28 Sep 2007 22:47:16 -0400
    Jeff Garzik <> 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 <>

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