Re: [PATCH 13/14] lockdep: Implement lock pinning

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 05, 2015 at 11:55:52AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > RFC: a possible alternative API would be something like:
> > 
> >   int cookie = lockdep_pin_lock(&foo);
> >   ...
> >   lockdep_unpin_lock(&foo, cookie);
> 
> Yeah, this would be even nicer.
> 
> > Where we pick a random number for the pin_count; this makes it
> > impossible to sneak a lock break in without also passing the right
> > cookie along.
> > 
> > I've not done this because it ends up generating code for !LOCKDEP,
> > esp. if you need to pass the cookie around for some reason.
> 
> The cookie could be a zero-size structure, which can be 'passed around' 
> syntactically but creates no overhead in the code.
> 
> But I'd expect cookie-passing to be a sign of badness in most cases: the lock 
> should generally be unpinned at the same level of abstraction...

I have tried to make this work, but so far I've failed at making the
!LOCKDEP case generate 'similar' code.

Esp, things like:

	rq = task_rq_lock(p, flags);

	...

	task_rq_unlock(rq, p, flags);

Need to somehow pass the cookie, and all pure stack based approaches
I've tried ended up being ugly and generating weird code.

So I'll keep the non-cookie approach for now.