| From: |
| Ingo Molnar <mingo-AT-kernel.org> |
| To: |
| Andrew Morton <akpm-AT-linux-foundation.org> |
| Subject: |
| Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t |
| Date: |
| Thu, 20 Jul 2017 11:34:02 +0200 |
| Message-ID: |
| <20170720093402.55alnsgsodgs4mfk@gmail.com> |
| Cc: |
| Davidlohr Bueso <dave-AT-stgolabs.net>, "Eric W. Biederman" <ebiederm-AT-xmission.com>, Elena Reshetova <elena.reshetova-AT-intel.com>, linux-kernel-AT-vger.kernel.org, peterz-AT-infradead.org, gregkh-AT-linuxfoundation.org, mingo-AT-redhat.com, adobriyan-AT-gmail.com, serge-AT-hallyn.com, arozansk-AT-redhat.com, keescook-AT-chromium.org, Hans Liljestrand <ishkamiel-AT-gmail.com>, David Windsor <dwindsor-AT-gmail.com> |
* Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> > On Wed, 19 Jul 2017, Andrew Morton wrote:
> >
> > >I do rather dislike these conversions from the point of view of
> > >performance overhead and general code bloat. But I seem to have lost
> > >that struggle and I don't think any of these are fastpath(?).
> >
> > Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
> > implementation), performance is supposed to be ok.
>
> Sure, things are OK for people who disable the feature.
So with the WIP fast-refcount series from Kees:
[PATCH v6 0/2] x86: Implement fast refcount overflow protection
I believe the robustness difference between optimized-refcount_t and
full-refcount_t will be marginal.
I.e. we'll be able to have both higher API safety _and_ performance.
> But for people who want to enable the feature we really should minimize the cost
> by avoiding blindly converting sites which simply don't need it: simple, safe,
> old, well-tested code. Why go and slow down such code? Need to apply some
> common sense here...
It's old, well-tested code _for existing, sane parameters_, until someone finds a
decade old bug in one of these with an insane parameters no-one stumbled upon so
far, and builds an exploit on top of it.
Only by touching all these places do we have a chance to improve things measurably
in terms of reducing the probability of bugs.
Thanks,
Ingo
