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

[Peter Zijlstra]

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?


> +static inline unsigned long get_bitaddr(int bitnum, unsigned long *addr)
> +{
> +	return (unsigned long)((char *)addr + bitnum);
> +}

And given you keep these lockdep_map thingies out-of-line, the original
structure remains dense and thus the above munging can easily result in
collisions.

Now, I suppose its rather unlikely, but given its entirely silent if it
happens, this is bad.