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SubjectRe: MSI interrupts and disable_irq
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 :)

Jeff


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