linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Date:   Wed, 31 Oct 2018 16:19:58 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation

On Wed, Oct 31, 2018 at 4:10 PM Igor Stoppa <igor.stoppa@...il.com> wrote:
>
>
>
> On 01/11/2018 00:57, Andy Lutomirski wrote:
> >
> >
> >> On Oct 31, 2018, at 2:00 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
>
>
> >> I _think_ the use-case for atomics is updating the reference counts of
> >> objects that are in this write-rare domain. But I'm not entirely clear
> >> on that myself either. I just really want to avoid duplicating that
> >> stuff.
> >
> > Sounds nuts. Doing a rare-write is many hundreds of cycles at best. Using that for a reference count sounds wacky.
> >
> > Can we see a *real* use case before we over complicate the API?
> >
>
>
> Does patch #14 of this set not qualify? ima_htable.len ?
>
> https://www.openwall.com/lists/kernel-hardening/2018/10/23/20
>

Do you mean this (sorry for whitespace damage):

+ pratomic_long_inc(&ima_htable.len);

- atomic_long_inc(&ima_htable.len);
  if (update_htable) {
    key = ima_hash_key(entry->digest);
-   hlist_add_head_rcu(&qe->hnext, &ima_htable.queue[key]);
+   prhlist_add_head_rcu(&qe->hnext, &ima_htable.queue[key]);
  }

ISTM you don't need that atomic operation -- you could take a spinlock
and then just add one directly to the variable.

--Andy