Re: [PATCH 04/21] Generic percpu refcounting

[Rusty Russell]

Kent Overstreet <koverstreet@xxxxxxxxxx> writes:
> This implements a refcount with similar semantics to
> atomic_get()/atomic_dec_and_test() - but percpu.

Ah!  This is why I was CC'd... Now I understand.  Thanks :)

Delighted to see someone chasing this.  I had an implementation of such
a thing last decade, but the slowmode pattern didn't make for trivial
kref conversions, so I dropped it.

Note: I haven't read the other feedback yet, so ignore if dups.

> +int percpu_ref_init(struct percpu_ref *ref);

Why not just run is slow mode when allocation fails?  Things which can't
fail make for simpler use.

> +int percpu_ref_tryget(struct percpu_ref *ref);
> +int percpu_ref_put_initial_ref(struct percpu_ref *ref);

This is part of a slightly different pattern: the owned refcount.

In fact, I think that's the most sane pattern to use (but I could be
wrong; does the AIO stuff fit?). If so, promote this to the first class
citizen, and if necessary expose kill as __percpu_ref_kill()?

(I might suggest percpu_ref_owner_put() as a name, in fact).

> +/**
> + * percpu_ref_get - increment a dynamic percpu refcount
> + *
> + * Analagous to atomic_inc().
> +  */
> +static inline void percpu_ref_get(struct percpu_ref *ref)
> +{
> +	unsigned __percpu *pcpu_count;
> +
> +	preempt_disable();
> +
> +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> +
> +	if (pcpu_count)
> +		__this_cpu_inc(*pcpu_count);
> +	else
> +		atomic_inc(&ref->count);
> +
> +	preempt_enable();
> +}

s/preempt_disable()/rcu_read_lock()/ ?

> +/**
> + * percpu_ref_put - decrement a dynamic percpu refcount
> + *
> + * Returns true if the result is 0, otherwise false; only checks for the ref
> + * hitting 0 after percpu_ref_kill() has been called. Analagous to
> + * atomic_dec_and_test().
> + */
> +static inline int percpu_ref_put(struct percpu_ref *ref)
> +{
> +	unsigned __percpu *pcpu_count;
> +	int ret = 0;
> +
> +	preempt_disable();
> +
> +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> +
> +	if (pcpu_count)
> +		__this_cpu_dec(*pcpu_count);
> +	else
> +		ret = atomic_dec_and_test(&ref->count);
> +
> +	preempt_enable();
> +
> +	return ret;
> +}

Here too.  And if you don't put unlikely() in this code, you lose kernel
hacker points :)

And int/true/false is for old-timers.

> +
> +unsigned percpu_ref_count(struct percpu_ref *ref);
> +int percpu_ref_kill(struct percpu_ref *ref);
> +
> +/**
> + * percpu_ref_dead - check if a dynamic percpu refcount is shutting down
> + *
> + * Returns true if percpu_ref_kill() has been called on @ref, false otherwise.
> + */
> +static inline int percpu_ref_dead(struct percpu_ref *ref)
> +{
> +	return ref->pcpu_count == NULL;
> +}

Can you unexpose these?  I think percpu_ref_init(), ...get(), ...put()
and ...put_initial() are a nicer API.

> +int percpu_ref_kill(struct percpu_ref *ref)
> +{
> +	unsigned __percpu *pcpu_count;
> +	unsigned __percpu *old;
> +	unsigned count = 0;
> +	int cpu;
> +
> +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> +
> +	do {
> +		if (!pcpu_count)
> +			return 0;
> +
> +		old = pcpu_count;
> +		pcpu_count = cmpxchg(&ref->pcpu_count, old, NULL);
> +	} while (pcpu_count != old);

This is more complex than it needs to be, no?


        pcpu_count = ACCESS_ONCE(ref->pcpu_count);
        if (!pcpu_count)
                return 0;
        if (cmpxchg(&ref->pcpu_count, pcpu_count, NULL) == NULL)
                return 0;

Of course, if all callers use the owner pattern, this is simply:

        pcpu_count = ACCESS_ONCE(ref->pcpu_count);
        BUG_ON(!pcpu_count);

> +	synchronize_sched();

synchronize_rcu() ?

> +	for_each_possible_cpu(cpu)
> +		count += *per_cpu_ptr(pcpu_count, cpu);
> +
> +	free_percpu(pcpu_count);
> +
> +	pr_debug("global %lli pcpu %i",
> +		 (int64_t) atomic_read(&ref->count), (int) count);
> +
> +	atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
> +
> +	return 1;
> +}
> +
> +/**
> + * percpu_ref_put_initial_ref - safely drop the initial ref
> + *
> + * A percpu refcount needs a shutdown sequence before dropping the initial ref,
> + * to put it back into single atomic_t mode with the appropriate barriers so
> + * that percpu_ref_put() can safely check for it hitting 0 - this does so.
> + *
> + * Returns true if @ref hit 0.
> + */
> +int percpu_ref_put_initial_ref(struct percpu_ref *ref)
> +{
> +	if (percpu_ref_kill(ref)) {
> +		return percpu_ref_put(ref);
> +	} else {
> +		WARN_ON(1);
> +		return 0;
> +	}
> +}

Note that percpu_ref_restore_initial_ref() is also possible, and may be
useful for the module code... (or percpu_ref_owner_get).

Great stuff!
Rusty.
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