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SubjectRe: [PATCH v3 6/8] locking/lockdep: Reuse freed chain_hlocks entries
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On 1/16/20 4:13 PM, Peter Zijlstra wrote:
> On Wed, Jan 15, 2020 at 04:43:11PM -0500, Waiman Long wrote:
>> +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.
>> + */
>> + for_each_chain_block(0, prev, curr, next) {
>> + 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;
>> + }
>> + }
>> + return -1;
>> +}
>> +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;
>> +}
> Argh, that's still _two_ half allocators.
>
> Here, please try this one, it seems to boot. It compiles with some
> noise, but that is because GCC is stupid and I'm too tired.
>
> ---
>
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1071,15 +1071,22 @@ static inline void check_data_structures
>
> #endif /* CONFIG_DEBUG_LOCKDEP */
>
> +static void init_chain_block_buckets(void);
> +
> /*
> * Initialize the lock_classes[] array elements, the free_lock_classes list
> * and also the delayed_free structure.
> */
> static void init_data_structures_once(void)
> {
> - static bool ds_initialized, rcu_head_initialized;
> + static bool ds_initialized, rcu_head_initialized, chain_block_initialized;
> int i;
>
> + if (!chain_block_initialized) {
> + chain_block_initialized = true;
> + init_chain_block_buckets();
> + }
> +
> if (likely(rcu_head_initialized))
> return;

Oh, I was not aware that there is such a init_data_structure_once()
function. I don't think we need a chain_block_initialized. The
ds_initialized should be enough and the init_chain_block_buckets() can
be put to the end of the function.

Other than that, the rests look OK to me so far. I will try it out tomorrow.

Thanks,
Longman

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