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

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  Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at  kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260 Sedat Dilek <sedat.dilek@gmail.com> - 2016-04-04 17:40 +0200
    Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at  kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260 Peter Zijlstra <peterz@infradead.org> - 2016-04-04 18:10 +0200

#1370696 — Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260

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.
>
> ---
>  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)
>  {

Will you push that also to peterz/queue.git#locking/urgent?

- Sedat -

P.S.: "lockdep: print chain_key collision information" in your above
tree differs from the upstream one.

[1] http://git.kernel.org/cgit/linux/kernel/git/peterz/queue.git/log/?h=locking/urgent

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#1370728

On Mon, Apr 04, 2016 at 05:31:40PM +0200, Sedat Dilek wrote:
> > +/* 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);
> > +}

> Will you push that also to peterz/queue.git#locking/urgent?

Only after I've had a play and observed it making any difference; Dmitry
supposedly has a reproducer, but I've not yet had a moment to try.

> - Sedat -
> 
> P.S.: "lockdep: print chain_key collision information" in your above
> tree differs from the upstream one.

I'll drop mine, I just hadn't moved to the very latestestest tip.

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