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Date: Tue, 17 Jan 2017 08:14:56 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: mingo@kernel.org, tglx@linutronix.de, walken@google.com,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, npiggin@gmail.com
Subject: Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature
Message-ID: <20170117071456.GK25813@worktop.programming.kicks-ass.net>
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On Tue, Jan 17, 2017 at 11:05:42AM +0900, Byungchul Park wrote:
> On Mon, Jan 16, 2017 at 04:10:01PM +0100, Peter Zijlstra wrote:
> > On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote:

> > > @@ -155,6 +164,9 @@ struct lockdep_map {
> > >  	int				cpu;
> > >  	unsigned long			ip;
> > >  #endif
> > > +#ifdef CONFIG_LOCKDEP_CROSSRELEASE
> > > +	struct cross_lock		*xlock;
> > > +#endif
> > 
> > The use of this escapes me; why does the lockdep_map need a pointer to
> > this?
> 
> Lockdep interfaces e.g. lock_acquire(), lock_release() and lock_commit()
> use lockdep_map as an arg, but crossrelease need to extract cross_lock
> instances from that.

> > Why not do something like:
> > 
> > struct lockdep_map_cross {
> > 	struct lockdep_map	map;
> > 	struct held_lock	hlock;
> > }

Using a structure like that, you can pass lockdep_map_cross around just
fine, since the lockdep_map is the first member, so the pointers are
interchangeable. At worst we might need to munge a few typecasts.

But then the cross release code can simply cast to the bigger type and
have access to the extra data it knows to be there.