In the comments section of a recent LWN article [1], Neil asked if we could have a way for refcount users to avoid getting to the saturated state if they have a way of handling fallback gracefully. Here's an attempt to provide that functionality. I'm not sure it's compatible with Kees' "x86/asm: Implement fast refcount overflow protection", but I thought I'd send it out anyway so people who have thought about this more deeply than I have can tell me if it's an idea worth pursuing or not.
[1] https://lwn.net/Articles/786044/ diff --git a/include/linux/refcount.h b/include/linux/refcount.h index e28cce21bad6..c4b15b5ec084 100644 --- a/include/linux/refcount.h +++ b/include/linux/refcount.h @@ -108,6 +108,21 @@ static inline void refcount_dec(refcount_t *r) # endif /* !CONFIG_ARCH_HAS_REFCOUNT */ #endif /* CONFIG_REFCOUNT_FULL */ +/** + * refcount_try_inc - Increment a refcount if it's below INT_MAX + * @r: the refcount to increment + * + * Avoid the counter saturating by declining to increment the counter + * if it is more than halfway to saturation. + */ +static inline __must_check bool refcount_try_inc(refcount_t *r) +{ + if (refcount_read(r) < 0) + return false; + refcount_inc(r); + return true; +} + extern __must_check bool refcount_dec_if_one(refcount_t *r); extern __must_check bool refcount_dec_not_one(refcount_t *r); extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);
