Re: [PATCH 2/9] percpu_ref: minor code and comment updates

[Kent Overstreet]

On Tue, Sep 23, 2014 at 01:55:11AM -0400, Tejun Heo wrote:
> * Some comments became stale.  Updated.
> * percpu_ref_tryget() unnecessarily initializes @ret.  Removed.
> * A blank line removed from percpu_ref_kill_rcu().
> * Explicit function name in a WARN format string replaced with __func__.
> * WARN_ON() in percpu_ref_reinit() converted to WARN_ON_ONCE().
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>

Reviewed-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>

> ---
>  include/linux/percpu-refcount.h | 25 ++++++++++++++++---------
>  lib/percpu-refcount.c           | 14 ++++++--------
>  2 files changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index f015f13..d44b027 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -115,8 +115,10 @@ static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
>   * percpu_ref_get - increment a percpu refcount
>   * @ref: percpu_ref to get
>   *
> - * Analagous to atomic_inc().
> -  */
> + * Analagous to atomic_long_inc().
> + *
> + * This function is safe to call as long as @ref is between init and exit.
> + */
>  static inline void percpu_ref_get(struct percpu_ref *ref)
>  {
>  	unsigned long __percpu *pcpu_count;
> @@ -138,12 +140,12 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
>   * Increment a percpu refcount unless its count already reached zero.
>   * Returns %true on success; %false on failure.
>   *
> - * The caller is responsible for ensuring that @ref stays accessible.
> + * This function is safe to call as long as @ref is between init and exit.
>   */
>  static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  {
>  	unsigned long __percpu *pcpu_count;
> -	int ret = false;
> +	int ret;
>  
>  	rcu_read_lock_sched();
>  
> @@ -166,12 +168,13 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>   * Increment a percpu refcount unless it has already been killed.  Returns
>   * %true on success; %false on failure.
>   *
> - * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
> - * will fail.  For such guarantee, percpu_ref_kill_and_confirm() should be
> - * used.  After the confirm_kill callback is invoked, it's guaranteed that
> - * no new reference will be given out by percpu_ref_tryget().
> + * Completion of percpu_ref_kill() in itself doesn't guarantee that this
> + * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
> + * should be used.  After the confirm_kill callback is invoked, it's
> + * guaranteed that no new reference will be given out by
> + * percpu_ref_tryget_live().
>   *
> - * The caller is responsible for ensuring that @ref stays accessible.
> + * This function is safe to call as long as @ref is between init and exit.
>   */
>  static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
>  {
> @@ -196,6 +199,8 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
>   *
>   * Decrement the refcount, and if 0, call the release function (which was passed
>   * to percpu_ref_init())
> + *
> + * This function is safe to call as long as @ref is between init and exit.
>   */
>  static inline void percpu_ref_put(struct percpu_ref *ref)
>  {
> @@ -216,6 +221,8 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
>   * @ref: percpu_ref to test
>   *
>   * Returns %true if @ref reached zero.
> + *
> + * This function is safe to call as long as @ref is between init and exit.
>   */
>  static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
>  {
> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> index 070dab5..8ef3f5c 100644
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -108,7 +108,6 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
>  	 * reaching 0 before we add the percpu counts. But doing it at the same
>  	 * time is equivalent and saves us atomic operations:
>  	 */
> -
>  	atomic_long_add((long)count - PCPU_COUNT_BIAS, &ref->count);
>  
>  	WARN_ONCE(atomic_long_read(&ref->count) <= 0,
> @@ -120,8 +119,8 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
>  		ref->confirm_kill(ref);
>  
>  	/*
> -	 * Now we're in single atomic_t mode with a consistent refcount, so it's
> -	 * safe to drop our initial ref:
> +	 * Now we're in single atomic_long_t mode with a consistent
> +	 * refcount, so it's safe to drop our initial ref:
>  	 */
>  	percpu_ref_put(ref);
>  }
> @@ -134,8 +133,8 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
>   * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
>   * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
>   * called after @ref is seen as dead from all CPUs - all further
> - * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
> - * for more details.
> + * invocations of percpu_ref_tryget_live() will fail.  See
> + * percpu_ref_tryget_live() for more details.
>   *
>   * Due to the way percpu_ref is implemented, @confirm_kill will be called
>   * after at least one full RCU grace period has passed but this is an
> @@ -145,8 +144,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
>  				 percpu_ref_func_t *confirm_kill)
>  {
>  	WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,
> -		  "percpu_ref_kill() called more than once on %pf!",
> -		  ref->release);
> +		  "%s called more than once on %pf!", __func__, ref->release);
>  
>  	ref->pcpu_count_ptr |= PCPU_REF_DEAD;
>  	ref->confirm_kill = confirm_kill;
> @@ -172,7 +170,7 @@ void percpu_ref_reinit(struct percpu_ref *ref)
>  	int cpu;
>  
>  	BUG_ON(!pcpu_count);
> -	WARN_ON(!percpu_ref_is_zero(ref));
> +	WARN_ON_ONCE(!percpu_ref_is_zero(ref));
>  
>  	atomic_long_set(&ref->count, 1 + PCPU_COUNT_BIAS);
>  
> -- 
> 1.9.3
> 
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