Re: [PATCH 1/5] lockdep: Implement bitlock map allocator

[Peter Zijlstra]

On Mon, Jul 04, 2016 at 04:29:25PM +0900, Byungchul Park wrote:
> On Fri, Jul 01, 2016 at 09:53:12AM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 01, 2016 at 09:24:44AM +0900, Byungchul Park wrote:
> > > On Thu, Jun 30, 2016 at 02:59:19PM +0200, Peter Zijlstra wrote:
> > > > On Mon, Jun 20, 2016 at 01:55:11PM +0900, Byungchul Park wrote:
> > > > 
> > > > > +struct bitlock_map {
> > > > > +	struct hlist_node	hash_entry;
> > > > > +	unsigned long		bitaddr; /* ID */
> > > > > +	struct lockdep_map	map;
> > > > > +	int			ref; /* reference count */
> > > > > +};
> > > > 
> > > > So this is effectively bigger than just adding a struct lockdep_map into
> > > > whatever structure holds the bit spinlock to begin with.
> > > > 
> > > > What is the gain?
> > > 
> > > 1. I don't want to make being aware of lockdep essential to user of
> > >    bit-base lock, like spin lock, mutex, semaphore ans so on. In other
> > >    words, I want to make it work transparently.
> > 
> > I want to discourage the use of bitlocks, they stink.
> 
> I agree it has some problems. But someone who are sensive to memory
> consumption still need to use bit-based lock. Right?
> 
> I can stop this proposal because it's meaningless if bit-based lock can be
> removed entirely since any requirement for bit-based lock does not exist
> at all. But IMHO, it's worthy if the requirement be.
> 
> > bitlocks must by their constraint be a test-and-set lock, with all the
> > known problems those have. It also means they're a royal pain for -rt.
> 
> I also think it's better to use rather spinlock in most cases unless memory
> consumption is critical problem. But in the case memory consumption is
> critical... what can we do?

So RT is already patching a whole bunch of bit-spinlocks into proper
spinlocks, I would much rather we do that and get lockdep coverage that
way.

That is, have the bit-spinlock for 'normal' kernels and use the proper
spinlock for LOCKDEP || PREEMPT_RT kernels.