linux-kernel - Re: [PATCH v6 6/6] locking/lockdep: Reuse freed chain

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Date:   Thu, 6 Feb 2020 12:06:45 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v6 6/6] locking/lockdep: Reuse freed chain_hlocks entries

On 2/6/20 11:03 AM, Peter Zijlstra wrote:
> On Thu, Feb 06, 2020 at 10:24:08AM -0500, Waiman Long wrote:
>> +#define for_each_chain_block(bucket, prev, curr)		\
>> +	for ((prev) = -1, (curr) = chain_block_buckets[bucket];	\
>> +	     (curr) >= 0;					\
>> +	     (prev) = (curr), (curr) = chain_block_next(curr))
>> +static inline void add_chain_block(int offset, int size)
>> +{
>> +	int bucket = size_to_bucket(size);
>> +	int next = chain_block_buckets[bucket];
>> +	int prev, curr;
>> +
>> +	if (unlikely(size < 2)) {
>> +		/*
>> +		 * We can't store single entries on the freelist. Leak them.
>> +		 *
>> +		 * One possible way out would be to uniquely mark them, other
>> +		 * than with CHAIN_BLK_FLAG, such that we can recover them when
>> +		 * the block before it is re-added.
>> +		 */
>> +		if (size)
>> +			nr_lost_chain_hlocks++;
>> +		return;
>> +	}
>> +
>> +	nr_free_chain_hlocks += size;
>> +	if (!bucket) {
>> +		nr_large_chain_blocks++;
>> +
>> +		if (unlikely(next >= 0)) {
> I was surprised by this condition..

Yes, this condition is optional and the code will still work as expected
without that. I added that so that for the common case where there is
only 1 chain block in block 0 and it gets deleted and added
repetitively, it will go to the simpler code path instead of the more
complicated one.

Cheers,
Longman