Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260

Sedat Dilek
2016-06-03T15:15:42Z

On 3/30/16, 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.
>
> ---
> 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