On Wed, Jul 29, 2020 at 01:11:20PM +0200, pet...@infradead.org wrote:
> Subject: locking/refcount: Provide __refcount API to obtain the old value
> From: Peter Zijlstra <pet...@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 <dhowe...@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@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?
