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Date: Thu, 11 Jun 2015 13:37:10 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: umgwanakikbuti@gmail.com, mingo@elte.hu, ktkhai@parallels.com,
        rostedt@goodmis.org, tglx@linutronix.de, juri.lelli@gmail.com,
        pang.xunlei@linaro.org, oleg@redhat.com, wanpeng.li@linux.intel.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 13/14] lockdep: Implement lock pinning
Message-ID: <20150611113710.GS19282@twins.programming.kicks-ass.net>
References: <20150605084836.364306429@infradead.org>
 <20150605085206.135690748@infradead.org>
 <20150605095552.GA7893@gmail.com>
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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.