linux-kernel - Re: [PATCH v2 4/6] locking/lockdep: Reuse freed chain

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Message-ID: <20200113155128.GX2844@hirez.programming.kicks-ass.net>
Date:   Mon, 13 Jan 2020 16:51:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v2 4/6] locking/lockdep: Reuse freed chain_hlocks entries

On Mon, Dec 16, 2019 at 10:15:15AM -0500, Waiman Long wrote:
> +#define CHAIN_HLOCKS_MASK	0xffff

> +static inline void set_chain_block(int offset, int size, int next)
> +{
> +	if (unlikely(offset < 0)) {
> +		chain_block_buckets[0] = next;
> +		return;
> +	}
> +	chain_hlocks[offset] = (next >> 16) | CHAIN_BLK_FLAG;
> +	chain_hlocks[offset + 1] = next & CHAIN_HLOCKS_MASK;
> +	if (size > MAX_CHAIN_BUCKETS) {
> +		chain_hlocks[offset + 2] = size >> 16;
> +		chain_hlocks[offset + 3] = size & CHAIN_HLOCKS_MASK;
> +	}
> +}

AFAICT HLOCKS_MASK is superfluous. That is, you're assigning to a u16,
it will truncate automagically.

But if you want to make it more explicit, something like:

  chain_hlocks[offset + 1] = (u16)next;

might be easier to read still.