linux-kernel - Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount

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Date:   Thu, 20 Jul 2017 11:34:02 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, mingo@...hat.com, adobriyan@...il.com,
        serge@...lyn.com, arozansk@...hat.com, keescook@...omium.org,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to
 refcount_t


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@...olabs.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