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

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Date:	Fri, 5 Jun 2015 11:55:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/14] lockdep: Implement lock pinning


* Peter Zijlstra <peterz@...radead.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...

Thanks,

	Ingo
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