Re: lockdep and threaded IRQs (was: ...)

On Mon, 2009-03-02 at 14:09 -0800, David Brownell wrote:

On Monday 02 March 2009, Peter Zijlstra wrote:

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How so?, its the natural extension of that work.

Not the work to shrink the amount of time IRQ latencies
by shrinking the amount of time IRQs are disabled by
IRQ handlers.

Ugh, that's done by pushing work out of the hardirq context,

That's one of many techniques currently used.

Tradeoffs don't always favor larger driver updates
and re-validation though.  Sometimes it's simpler
to just leverage the reality that "hardirq context"
does not require using IRQF_DISABLED.

Simpler doesn't mean its a good idea, it opens the door to stack
overruns for one.

Its very unfortunate that people think its a good idea.

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not by doing silly things like enabling irqs from hardirq context.

Somehow I'm certain you have NOT analysed every one of the
thousands of IRQ handlers in various Linux drivers to know
with certainty that's not the reason IRQ_DISABLED is cleared.

There are also *other* reasons to leave IRQ_DISABLED clear.

There might be reasons to do so, that doesn't mean its a good idea and
the desired thing to do.

I state that every !IRQF_DISABLED usage is a bug, either due to broken
hardware or broken drivers.