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Date: Wed, 30 Mar 2016 17:21:32 +0200
Message-ID: <CA+icZUUvEWoAQUUXPxPxzb-Y7ajuLOJqXFF2sfR1iBbQu0yBoA@mail.gmail.com>
Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at
 kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260
From: Sedat Dilek <sedat.dilek@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
        Alfredo Alvarez Fernandez <alfredoalvarezfernandez@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Mar 30, 2016 at 4:06 PM, Peter Zijlstra <peterz@infradead.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.
>

Is this patch in combination with the previous one?

- Sedat -

> ---
>  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)
>  {