linux-kernel - Re: [PATCH 1/2] <linux/hash.h>: Make hash_64(), hash

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Message-ID: <20160502132804.GF12845@twins.programming.kicks-ass.net>
Date:	Mon, 2 May 2016 15:28:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	George Spelvin <linux@...izon.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	torvalds@...ux-foundation.org, bfields@...ldses.org,
	eric.dumazet@...il.com, jlayton@...chiereds.net,
	linux-nfs@...r.kernel.org, riel@...hat.com
Subject: Re: [PATCH 1/2] <linux/hash.h>: Make hash_64(), hash_ptr() return 32
 bits

On Mon, May 02, 2016 at 06:20:16AM -0400, George Spelvin wrote:
> Subject: [PATCH 1/2] <linux/hash.h>: Make hash_64(), hash_ptr() return 32 bits

> +static __always_inline u64 hash_64(u64 val, unsigned bits)
> +{
>  	/* High bits are more random, so use them. */
> +	return __hash_64(val) >> (64 - bits);
>  }

Is the subject stale or the above a mistake? Because hash_64() still
very much seems to return u64.

Also, I think I would prefer to keep it like this, I would like to use
it for kernel/locking/lockdep.c:iterate_chain_key(), which currently is
a somewhat crap hash.

Something like:

static inline u64 iterate_chain_key(u64 key1, u64 key2)
{
	return hash_64(key1 ^ key2, 64);
}