linux-kernel - Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock

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Message-ID: <CA+icZUUgZvQnMwsnaKcos259nOQuDN2sU1EmRkZGGmtUsFh-AQ@mail.gmail.com>
Date:	Fri, 3 Jun 2016 17:15:33 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Alfredo Alvarez Fernandez <alfredoalvarezfernandez@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at
 kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260

On 3/30/16, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Mar 30, 2016 at 11:36:59AM +0200, Peter Zijlstra wrote:
>> Furthermore, our hash function has definite room for improvement.
>
> After a bit of reading, using a 'strong' PRNG as base for a hash
> function seems generally suggested.
>
> ---
>  kernel/locking/lockdep.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 53ab2f85d77e..0f7dba4144d6 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -308,10 +308,21 @@ static struct hlist_head
> chainhash_table[CHAINHASH_SIZE];
>   * It's a 64-bit hash, because it's important for the keys to be
>   * unique.
>   */
> -#define iterate_chain_key(key1, key2) \
> -	(((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
> -	((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
> -	(key2))
> +
> +/* https://en.wikipedia.org/wiki/Xorshift#xorshift.2A */
> +#define UINT64_C(x) x##ULL
> +static inline u64 xorshift64star(u64 x)
> +{
> +	x ^= x >> 12; // a
> +	x ^= x << 25; // b
> +	x ^= x >> 27; // c
> +	return x * UINT64_C(2685821657736338717);
> +}
> +
> +static inline u64 iterate_chain_key(u64 hash, u64 class_idx)
> +{
> +	return xorshift64star(hash ^ class_idx);
> +}
>
>  void lockdep_off(void)
>  {
>

You dropped this implementation and the below is a different approach?

"locking/lockdep: Use __jhash_mix() for iterate_chain_key()"

- Sedat -

[1] http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=locking/core&id=724e66913097c2eba029b06dcd05c5a2be78d466