linux-kernel - Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount

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Message-ID: <20170901133644.jf57pwuaep6zirxz@hirez.programming.kicks-ass.net>
Date:   Fri, 1 Sep 2017 15:36:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "tj@...nel.org" <tj@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "lizefan@...wei.com" <lizefan@...wei.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "eparis@...hat.com" <eparis@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "luto@...nel.org" <luto@...nel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dvhart@...radead.org" <dvhart@...radead.org>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>
Subject: Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to
 refcount_t

On Fri, Sep 01, 2017 at 01:24:16PM +0000, Reshetova, Elena wrote:
> 
> > On Fri, Sep 01, 2017 at 11:05:33AM +0000, Reshetova, Elena wrote:
> > > Actually on the second thought: does the above memory ordering differences
> > > really apply when  we have ARCH_HAS_REFCOUNT? To me it looks like the way
> > > how it is currently implemented for x86 is the same way as it is for atomic cases.
> > 
> > Never look to x86 for memory ordering, its boring.
> > 
> > And yes, for the ARM implementation it can certainly make a difference.
> 
> So, yes, what I am trying to say is that it can really depend if you have ARCH_HAS_REFCOUNT
> enabled or not and then also based on architecture. Thus I believe is also true for atomic: there
> might be differences when you use arch. dependent version of function or not. 

So the generic one in lib/refcount.c is already weaker on ARM, they
don't need to do a ARCH specific 'fast' implementation for the
difference to show up.