linux-kernel - Re: Regression introduced by bfcfaa77bdf0f775263e906015982a608df01c76 (vfs: use 'unsigned long' accesses for dcache name comparison and hashing)

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Message-ID: <CA+55aFyiNKdkkvK0FH1ibo1iJj1HF9RjLEw3VOEjVwc0eiX8Sw@mail.gmail.com>
Date:	Thu, 22 Mar 2012 13:38:28 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: Regression introduced by bfcfaa77bdf0f775263e906015982a608df01c76
 (vfs: use 'unsigned long' accesses for dcache name comparison and hashing)

On Thu, Mar 22, 2012 at 1:24 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> OK, full_name_hash()/hash_name() definitely have a mismatch and it's on the
> names of length 8*n: trivial experiment shows that we have
> name hash_name full_name_hash

Good catch, guys.

Ugh. And I never noticed despite having run this code on my machines
for several weeks, because I don't think I have anything that uses the
"full_name_hash()" function. And it looked so obviously the same.

> Linus, which way do you prefer to shift it?  Should hash_name() change to
> match full_name_hash() or should it be the other way round?
>
> What happens is that you get multiplication by 9 and adding 0 in the former,
> after having added the last full word.  In the latter we add the last full
> word, see that there's nothing left and bugger off.

Yes. I think we should make things match "hash_name()", because that's
the one that is critical and we want to really generate good code for.

I think you can just move the "*=9" down in full_name_hash(), so that
we always "pre-multiply" the hash for the next round.  But I'll have
to double-check my logic.

                 Linus
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