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Date:   Wed, 29 Jul 2020 13:37:31 +0200
From:   peterz@infradead.org
To:     David Howells <dhowells@redhat.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        David Windsor <dwindsor@gmail.com>,
        Hans Liljestrand <ishkamiel@gmail.com>,
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        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,
        Will Deacon <will@kernel.org>
Subject: Re: [RFC][PATCH] locking/refcount: Provide __refcount API to obtain
 the old value
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On Wed, Jul 29, 2020 at 01:11:20PM +0200, peterz@infradead.org wrote:

> Subject: locking/refcount: Provide __refcount API to obtain the old value
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Wed Jul 29 13:00:57 CEST 2020
> 
> David requested means to obtain the old/previous value from the
> refcount API for tracing purposes.
> 
> Duplicate (most of) the API as __refcount*() with an additional
> 'int *' argument into which, if !NULL, the old value will be stored.
> 
> Requested-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/refcount.h |   65 +++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 57 insertions(+), 8 deletions(-)
> 
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -165,7 +165,7 @@ static inline unsigned int refcount_read
>   *
>   * Return: false if the passed refcount is 0, true otherwise
>   */
> -static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
> +static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
>  {
>  	int old = refcount_read(r);
>  
> @@ -174,12 +174,20 @@ static inline __must_check bool refcount
>  			break;
>  	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
>  
> +	if (oldp)
> +		*oldp = old;
> +
>  	if (unlikely(old < 0 || old + i < 0))
>  		refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
>  
>  	return old;
>  }
>  
> +static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
> +{
> +	return __refcount_add_not_zero(i, r, NULL);
> +}

so, I could also emulate C++'s

bool refcount_add_not_zero(int i, refcount_t *r, int *oldp = NULL)

style by going to town on this with a bunch of CPP magic, but I don't
think that'll actually make things clearer.

It'll look something like:

#define __REF_ARGS(_0, _1, _2, _3, _n, X...) _n
#define REG_ARGS(X...) __REF_ARGS(, ##X, 3, 2, 1, 0)

#define __REF_CONCAT(a, b) a ## b
#define REF_CONCAT(a, b) __REF_CONCAT(a, b)

#define REF_UNARY_2(func, arg1, arg2)	func(arg1, arg2)
#define REF_UNARY_1(func, arg1)		func(arg1, NULL)
#define REF_UNARY(func, X...) REF_CONCAT(REF_UNARY_, REF_ARGS(X))(func, X)

#define REF_BINARY_3(func, arg1, arg2, arg3)	func(arg1, arg2, arg3)
#define REF_BINARY_2(func, arg1, arg2)		func(arg1, arg2, NULL)
#define REF_BINARY(func, X...) REF_CONCAT(REF_BINARY_, REF_ARGS(X))(func, X)

#define refcount_add(X...)	REF_BINARY(__refcount_add, X)
#define refcount_inc(X...)	REF_UNARY(__refcount_inc, X)

Opinions?