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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
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