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

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Date:	Thu, 30 Jun 2016 14:59:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, npiggin@...e.de,
	sergey.senozhatsky@...il.com, gregkh@...uxfoundation.org,
	minchan@...nel.org
Subject: Re: [PATCH 1/5] lockdep: Implement bitlock map allocator

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.