Re: [PATCH 20/27] locking/lockdep: Free lock classes that are no longer in use

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 28, 2018 at 03:43:18PM -0800, Bart Van Assche wrote:
> +/* Must be called with the graph lock held. */
> +static void remove_class_from_lock_chain(struct lock_chain *chain,
> +					 struct lock_class *class)
> +{
> +	u64 chain_key;
> +	int i;
> +
> +	for (i = chain->base; i < chain->base + chain->depth; i++) {
> +		if (chain_hlocks[i] != class - lock_classes)
> +			continue;
> +		if (--chain->depth == 0)
> +			break;
> +		memmove(&chain_hlocks[i], &chain_hlocks[i + 1],
> +			(chain->base + chain->depth - i) *
> +			sizeof(chain_hlocks[0]));
> +		/*
> +		 * Each lock class occurs at most once in a
> +		 * lock chain so once we found a match we can
> +		 * break out of this loop.
> +		 */
> +		break;
> +	}
> +	/*
> +	 * Note: calling hlist_del_rcu() from inside a
> +	 * hlist_for_each_entry_rcu() loop is safe.
> +	 */
> +	if (chain->depth == 0) {
> +		/* To do: decrease chain count. See also inc_chains(). */
> +		hlist_del_rcu(&chain->entry);
> +		return;
> +	}
> +	chain_key = 0;
> +	for (i = chain->base; i < chain->base + chain->depth; i++)
> +		chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
> +	if (chain->chain_key == chain_key)
> +		return;
> +	hlist_del_rcu(&chain->entry);
> +	chain->chain_key = chain_key;
> +	hlist_add_head_rcu(&chain->entry, chainhashentry(chain_key));
> +}
> +
> +/* Must be called with the graph lock held. */
> +static void remove_class_from_lock_chains(struct lock_class *class)
> +{
> +	struct lock_chain *chain;
> +	struct hlist_head *head;
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(chainhash_table); i++) {
> +		head = chainhash_table + i;
> +		hlist_for_each_entry_rcu(chain, head, entry) {
> +			remove_class_from_lock_chain(chain, class);
> +		}
> +	}
> +}

*shudder*, I suppose that is the reason I never went there.

I suoppose that if you don't do this too often it doesn't matter it is
horribly epxneisve.