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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.

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