linux-kernel - Re: [PATCH resend] audit: fix mark refcounting

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Message-ID: <CA+55aFz6vpv01OAGcq9n8Q4bVgihqSiwz2qwNjBAHeFFvUD5Hg@mail.gmail.com>
Date:	Thu, 15 Dec 2011 14:34:32 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Paris <eparis@...hat.com>
Cc:	Lino Sanfilippo <LinoSanfilippo@....de>,
	Miklos Szeredi <miklos@...redi.hu>,
	Al Viro <viro@...iv.linux.org.uk>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] audit: fix mark refcounting

On Thu, Dec 15, 2011 at 2:28 PM, Eric Paris <eparis@...hat.com> wrote:
>
> How expensive is an atomic_inc()/atomic_dec() combo?  If it's mostly
> free we can do it in the right place.

It's pretty expensive, but it depends a lot on cache details etc. It
involves a memory barrier on x86 too, and depending on architecture it
might be anything from 150 cycles (P4 - but by now probably nobody
cares) to "a few tens" of cycles (roughly 10-35 on modern x86).

The cache miss itself - if it happens - is not counted in the above cost.

A totally uncontended spinlock is actually cheaper than a pair of
atomic ops. So if it's a hot path it probably should be moved
elsewhere if possible.

                       Linus
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