Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

Vlastimil Babka
2025-01-08T09:16:11Z

On 12/26/24 18:07, Suren Baghdasaryan wrote:
> Introduce functions to increase refcount but with a top limit above
> which they will fail to increase. Setting the limit to 0 indicates
> no limit.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> include/linux/refcount.h | 20 +++++++++++++++++++-
> 1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index 35f039ecb272..e51a49179307 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -137,13 +137,19 @@ static inline unsigned int refcount_read(const refcount_t *r)
> }
>
> static inline __must_check __signed_wrap
> -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> +bool __refcount_add_not_zero_limited(int i, refcount_t *r, int *oldp,
> + int limit)
> {
> int old = refcount_read(r);
>
> do {
> if (!old)
> break;
> + if (limit && old + i > limit) {

Should this be e.g. "old > limit - i" to avoid overflow and false negative
if someone sets limit close to INT_MAX?

> + if (oldp)
> + *oldp = old;
> + return false;
> + }
> } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
>
> if (oldp)
> @@ -155,6 +161,12 @@ bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> return old;
> }
>
> +static inline __must_check __signed_wrap
> +bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> +{
> + return __refcount_add_not_zero_limited(i, r, oldp, 0);
> +}
> +
> /**
> * refcount_add_not_zero - add a value to a refcount unless it is 0
> * @i: the value to add to the refcount
> @@ -213,6 +225,12 @@ static inline void refcount_add(int i, refcount_t *r)
> __refcount_add(i, r, NULL);
> }
>
> +static inline __must_check bool __refcount_inc_not_zero_limited(refcount_t *r,
> + int *oldp, int limit)
> +{
> + return __refcount_add_not_zero_limited(1, r, oldp, limit);
> +}
> +
> static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
> {
> return __refcount_add_not_zero(1, r, oldp);