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From:   David Howells <dhowells@redhat.com>
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To:     Will Deacon <will@kernel.org>
Cc:     dhowells@redhat.com, peterz@infradead.org,
        Kees Cook <keescook@chromium.org>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        David Windsor <dwindsor@gmail.com>,
        Hans Liljestrand <ishkamiel@gmail.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Paul Moore <paul@paul-moore.com>, edumazet@google.com,
        paulmck@kernel.org, shakeelb@google.com,
        James Morris <jamorris@linux.microsoft.com>,
        alex.huangjianhui@huawei.com, dylix.dailei@huawei.com,
        chenzefeng2@huawei.com, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] locking/refcount: Provide __refcount API to obtain the old value
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Date:   Tue, 26 Jan 2021 11:25:39 +0000
Message-ID: <2697076.1611660339@warthog.procyon.org.uk>
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Will Deacon <will@kernel.org> wrote:

> > @@ -219,9 +235,14 @@ static inline void refcount_add(int i, r
> >   *
> >   * Return: true if the increment was successful, false otherwise
> >   */
> > +static inline __must_check bool __refcount_inc_not_zero(refcount_t *r=
, int *oldp)
> > +{
> > +	return __refcount_add_not_zero(1, r, oldp);
> > +}
> =

> Where returning both a bool to indicate whether the old value was zero
> and also the old value itself is a bit OTT.

Actually, with the i386 cmpxchg, that makes sense.  You can use the Z flag=
 to
give you the bool, saving on checking the old value.

David