Messages in this thread | | | Date | Thu, 27 Dec 2012 14:47:49 +0100 | Subject | Re: [PATCH 23/26] Generic dynamic per cpu refcounting | From | Fubo Chen <> |
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On Mon, Dec 3, 2012 at 9:58 PM, Kent Overstreet <koverstreet@google.com> wrote: > +#define pr_fmt(fmt) "%s: " fmt "\n", __func__
Dynamic debug already allows to insert the function name. Please consider leaving this line out entirely and move the "\n" to the end of individual printed lines.
> +#define PCPU_REF_PTR 0 > +#define PCPU_REF_NONE 1 > +#define PCPU_REF_DYING 2 > +#define PCPU_REF_DEAD 3
Why has "define" been used here instead of an enum ? Using "define" prevents the compiler to verify whether all possible values have been handled in a switch statement.
> +#define REF_STATUS(count) ((unsigned long) count & PCPU_STATUS_MASK)
Please leave out the cast in the above expression. It prevents that the compiler performs type checking on "count". Also, why is REF_STATUS() a define and not an inline function ?
> +struct percpu_ref { > + atomic64_t count; > + unsigned __percpu *pcpu_count; > +};
pcpu_count can either be a jiffies value (unsigned long) or a pointer to unsigned __percpu. So please consider using a union here. Has this code been checked with sparse ? I'm afraid that the above structure definition will confuse sparse a lot.
> + if (now - last <= HZ << PCPU_STATUS_BITS) {
Please consider changing "HZ" into a symbolic name that makes it clear that this value is the threshold in jiffies above which the switch is made from a single atomic_t refcount to the per-cpu refcounting scheme.
> +void __percpu_ref_get(struct percpu_ref *ref, bool alloc) > +{ > + unsigned __percpu *pcpu_count; > + uint64_t v; > + > + pcpu_count = rcu_dereference(ref->pcpu_count); > + > + if (REF_STATUS(pcpu_count) == PCPU_REF_PTR) { > + __this_cpu_inc(*pcpu_count); > + } else { > + v = atomic64_add_return(1 + (1ULL << PCPU_COUNT_BITS), > + &ref->count); > + > + if (!(v >> PCPU_COUNT_BITS) && > + REF_STATUS(pcpu_count) == PCPU_REF_NONE && alloc) > + percpu_ref_alloc(ref, pcpu_count); > + } > +}
What will happen if another thread invokes percpu_ref_kill() after the above rcu_dereference() call finished but before REF_STATUS() got invoked ? Can that percpu_ref_kill() finish before __this_cpu_inc(*pcpu_count) gets invoked ? Documentation/RCU/checklist.txt says: "All RCU list-traversal primitives, which include rcu_dereference(), list_for_each_entry_rcu(), and list_for_each_safe_rcu(), must be either within an RCU read-side critical section or must be protected by appropriate update-side locks."
> +int percpu_ref_kill(struct percpu_ref *ref)
The meaning of the return value of this function has not been documented.
Thanks,
Fubo.
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