linux-kernel - Re: [PATCH v5 1/3] refcount: Create unchecked atomic

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Message-ID: <20170531110918.tgbppvxgkkhoaclk@hirez.programming.kicks-ass.net>
Date:   Wed, 31 May 2017 13:09:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "arozansk@...hat.com" <arozansk@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 1/3] refcount: Create unchecked atomic_t implementation

On Wed, May 31, 2017 at 10:45:09AM +0000, Reshetova, Elena wrote:

> > +static inline __must_check bool refcount_add_not_zero(unsigned int i,
> > +
> > 	      refcount_t *r)
> > +{
> > +	return atomic_add_return(i, &r->refs) != 0;
> > +}
> 
> Maybe atomic_add_unless(&r->refs, i, 0) in order to be consistent with the below inc_not_zero implementation?

Yes, atomic_add_return() is strictly incorrect here since the add is
unconditional.

> > +static inline __must_check bool refcount_sub_and_test(unsigned int i,
> > +
> > 	      refcount_t *r)
> > +{
> > +	return atomic_sub_return(i, &r->refs) == 0;
> > +}
> 
> Any reason for not using atomic_sub_and_test() here?

> > +static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> > +{
> > +	return atomic_dec_return(&r->refs) == 0;
> > +}
> 
> Same here: atomic_dec_and_test()?

Both those are better because they return condition codes generated from
the operand itself.