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SubjectRe: [DOCUMENTATION] Revised Unreliable Kernel Locking Guide
On Fri, Dec 12, 2003 at 04:24:18PM +1100, Rusty Russell wrote:
> OK, I've put the html version up for your reading pleasure: the diff
> is quite extensive and hard to read.
> Feedback welcome,
> Rusty.
> --
> Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

Hello, Rusty,

Good stuff! A few comments, as always!

Thanx, Paul


o Hardware Interrupt / Hardware IRQ: How does in_irq()
know that interrupts have been blocked? The local_irq_disable()
does not seem to mess with the counter, and preempt_disable()
just does the standard inc/dec stuff...

o in_irq() is hardirq_count().
o hardirq_count() is (preempt_count() & HARDIRQ_MASK).
o preempt_count is an integer, HARDIRQ_MASK is a constant that
is out of the normal inc/dec range.

I see how an interrupt handler causes in_irq() to do its thing
via the irq_enter() and irq_exit() macros, but I don't see how
masking interrupts makes this happen.

Probably just my confusion, but...

Ditto for "in_interrupt()". That would be for both the
analysis and the probable confusion on my part.

o Software Interrupt / softirq: formatting botch "of'software".
This would be "o'software", right?

o Preemption: Would it be worth changing the first bit
of the second sentence to read something like: "In 2.5.4
and later, when CONFIG_PREEMPT is set, this changes:"?

Overzealous Prevention Of Deadlocks: Cute!!!

Avoiding Locks: Read Copy Update

o Might be worth noting explicitly early on that updaters are
running concurrently with readers. Should be obvious given
that the readers aren't doing any explicit synchronization,
but I have run into confusion on this point surprisingly often.

o Please add a note to the list_for_each_entry_rcu() description
saying that writers (who hold appropriate locks) need not use
the _rcu() variant.

o Don't understand the blank line before and after the
"struct rcu_head rcu;", but clearly doesn't affect
functionality. ;-)

o If nothing blocks between the call to __cache_find() and the
eventual object_put(), it is worthwhile to avoid the
reference-count manipulation. This would make all of
cache_find() be almost as fast as UP, rather than just
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