Re: [PATCH v2 4/6] locking/lockdep: Reuse freed chain_hlocks entries

[Peter Zijlstra]

On Mon, Dec 16, 2019 at 10:15:15AM -0500, Waiman Long wrote:
> +/*
> + * Return offset of a chain block of the right size or -1 if not found.
> + */
> +static inline int alloc_chain_hlocks_from_buckets(int size)
> +{
> +	int prev, curr, next;
> +
> +	if (!nr_free_chain_hlocks)
> +		return -1;
> +
> +	if (size <= MAX_CHAIN_BUCKETS) {
> +		curr = chain_block_buckets[size - 1];
> +		if (curr < 0)
> +			return -1;
> +
> +		chain_block_buckets[size - 1] = next_chain_block(curr);
> +		nr_free_chain_hlocks -= size;
> +		return curr;
> +	}
> +
> +	/*
> +	 * Look for a free chain block of the given size
> +	 *
> +	 * It is rare to have a lock chain with depth > MAX_CHAIN_BUCKETS.
> +	 * It is also more expensive as we may iterate the whole list
> +	 * without finding one.
> +	 */
> +	prev = -1;
> +	curr = chain_block_buckets[0];
> +	while (curr >= 0) {
> +		next = next_chain_block(curr);
> +		if (chain_block_size(curr) == size) {
> +			set_chain_block(prev, 0, next);
> +			nr_free_chain_hlocks -= size;
> +			nr_large_chain_blocks--;
> +			return curr;
> +		}
> +		prev = curr;
> +		curr = next;
> +	}
> +	return -1;
> +}

> +/*
> + * The graph lock must be held before calling this function.
> + *
> + * Return: an offset to chain_hlocks if successful, or
> + *	   -1 with graph lock released
> + */
> +static int alloc_chain_hlocks(int size)
> +{
> +	int curr;
> +
> +	if (size < 2)
> +		size = 2;
> +
> +	curr = alloc_chain_hlocks_from_buckets(size);
> +	if (curr >= 0)
> +		return curr;
> +
> +	BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
> +	BUILD_BUG_ON((1UL << 6)  <= ARRAY_SIZE(current->held_locks));
> +	BUILD_BUG_ON((1UL << 8*sizeof(chain_hlocks[0])) <=
> +		     ARRAY_SIZE(lock_classes));
> +
> +	/*
> +	 * Allocate directly from chain_hlocks.
> +	 */
> +	if (likely(nr_chain_hlocks + size <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
> +		curr = nr_chain_hlocks;
> +		nr_chain_hlocks += size;
> +		return curr;
> +	}
> +	if (!debug_locks_off_graph_unlock())
> +		return -1;
> +
> +	print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!");
> +	dump_stack();
> +	return -1;
> +}

*groan*....

That's _two_ allocators :/ And it can trivially fail, even if there's
plenty space available.

Consider nr_chain_hlocks is exhaused, and @size is empty, but size+1
still has blocks.

I'm guessing you didn't make it a single allocator because you didn't
want to implement block splitting? why?