linux-kernel - Re: [PATCH 4/4] locking/lockdep: Test all incompatible scenario at once in check_irq

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Date:   Tue, 9 Apr 2019 15:03:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 4/4] locking/lockdep: Test all incompatible scenario at
 once in check_irq_usage()

On Tue, Apr 02, 2019 at 06:02:44PM +0200, Frederic Weisbecker wrote:
> @@ -1988,45 +1961,151 @@ static int exclusive_bit(int new_bit)
>  	return state | (dir ^ LOCK_USAGE_DIR_MASK);
>  }
>  
> +static unsigned long exclusive_dir_mask(unsigned long mask)

Would you mind terribly if I call that: invert_dir_mask() ?

> +{
> +	unsigned long excl;
> +
> +	/* Invert dir */
> +	excl = (mask & LOCKF_ENABLED_IRQ_ALL) >> LOCK_USAGE_DIR_MASK;
> +	excl |= (mask & LOCKF_USED_IN_IRQ_ALL) << LOCK_USAGE_DIR_MASK;
> +
> +	return excl;
> +}
> +
> +static unsigned long exclusive_mask(unsigned long mask)
> +{
> +	unsigned long excl = exclusive_dir_mask(mask);
> +
> +	/* Strip read */
> +	excl |= (excl & LOCKF_IRQ_READ) >> LOCK_USAGE_READ_MASK;
> +	excl &= ~LOCKF_IRQ_READ;
> +
> +	return excl;
> +}

And I might write a comment to go with those functions; they're too
clever by half. I'm sure I'll have forgotten how they work in a few
months time.

Very well done :-)

> +/*
> + * Find the first pair of bit match between an original
> + * usage mask and an exclusive usage mask.
> + */
> +static int find_exclusive_match(unsigned long mask,
> +				unsigned long excl_mask,
> +				enum lock_usage_bit *bit,
> +				enum lock_usage_bit *excl_bit)
> +{
> +	int fs, nr = 0;
> +
> +	while ((fs = ffs(mask))) {
> +		int excl;
> +
> +		nr += fs;
> +		excl = exclusive_bit(nr - 1);
> +		if (excl_mask & lock_flag(excl)) {
> +			*bit = nr - 1;
> +			*excl_bit = excl;
> +			return 0;
> +		}
> +		mask >>= fs - 1;
> +		/*
> +		 * Prevent from shifts of sizeof(long) which can
> +		 * give unpredictable results.
> +		 */
> +		mask >>= 1;
> +	}
> +	return -1;

Should we write that like:

	for_each_set_bit(bit, &mask, LOCK_USED) {
		int excl = exclusive_bit(bit);
		if (excl_mask & lock_flag(excl)) {
			*bitp = bit;
			*excl_bitp = excl;
			return 0;
		}
	}
	return -1;

Or something along those lines?

> +}
> +