Re: [PATCH 1/2] percpu_ref: add percpu_ref_atomic_count()

[Dennis Zhou <dennis@kernel.org>]

Hello,

On Fri, Apr 16, 2021 at 01:22:51AM +0100, Pavel Begunkov wrote:
> Add percpu_ref_atomic_count(), which returns number of references of a
> percpu_ref switched prior into atomic mode, so the caller is responsible
> to make sure it's in the right mode.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  include/linux/percpu-refcount.h |  1 +
>  lib/percpu-refcount.c           | 26 ++++++++++++++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index 16c35a728b4c..0ff40e79efa2 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -131,6 +131,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
>  void percpu_ref_resurrect(struct percpu_ref *ref);
>  void percpu_ref_reinit(struct percpu_ref *ref);
>  bool percpu_ref_is_zero(struct percpu_ref *ref);
> +unsigned long percpu_ref_atomic_count(struct percpu_ref *ref);
>  
>  /**
>   * percpu_ref_kill - drop the initial ref
> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> index a1071cdefb5a..56286995e2b8 100644
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -425,6 +425,32 @@ bool percpu_ref_is_zero(struct percpu_ref *ref)
>  }
>  EXPORT_SYMBOL_GPL(percpu_ref_is_zero);
>  
> +/**
> + * percpu_ref_atomic_count - returns number of left references
> + * @ref: percpu_ref to test
> + *
> + * This function is safe to call as long as @ref is switch into atomic mode,
> + * and is between init and exit.
> + */
> +unsigned long percpu_ref_atomic_count(struct percpu_ref *ref)
> +{
> +	unsigned long __percpu *percpu_count;
> +	unsigned long count, flags;
> +
> +	if (WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count)))
> +		return -1UL;
> +
> +	/* protect us from being destroyed */
> +	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
> +	if (ref->data)
> +		count = atomic_long_read(&ref->data->count);
> +	else
> +		count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;

Sorry I missed Jens' patch before and also the update to percpu_ref.
However, I feel like I'm missing something. This isn't entirely related
to your patch, but I'm not following why percpu_count_ptr stores the
excess count of an exited percpu_ref and doesn't warn when it's not
zero. It seems like this should be an error if it's not 0?

Granted we have made some contract with the user to do the right thing,
but say someone does mess up, we don't indicate to them hey this ref is
actually dead and if they're waiting for it to go to 0, it never will.

> +	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
> +
> +	return count;
> +}
> +
>  /**
>   * percpu_ref_reinit - re-initialize a percpu refcount
>   * @ref: perpcu_ref to re-initialize
> -- 
> 2.24.0
> 

Thanks,
Dennis