Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2013-03-26 at 11:19 -0400, Rik van Riel wrote:
> That makes me wonder, how did mm_take_all_locks used to work before
> we turned the anon_vma lock into a mutex?
> 
> The code used to use spin_lock_nest_lock, but still has the potential
> to overflow the preempt counter. How did that ever work right?

It did trigger a bunch of warnings, but early on it was understood that
KVM would have 'few' vmas when starting and registering the
mmu_notifier thing.. then KVM bloated into insanity.

But aside from the warnings, if you overflow the regular preempt_count
bits, nothing really bad happens because you start poking at softirq
nesting, then hardirq etc.. all of those also disable preemption. 

You'll get a few 'unexpected' side-effects for things like
serving_softirq()/in_irq() or whatever those functions are called, but
other than that things mostly work.

I don't particularly like overflowing preempt count, but its mostly
harmless (up to a point). The much worse offender in my book is the
duration of the preempt_disable section thus created.

Esp with everything in user control, you can basically create an
arbitrary long non-preempt section with the semops.