[lttng-dev] [PATCH] Fix: revise urcu_read_lock_update() comment

[Mathieu Desnoyers]

On 6/13/23 11:45, Li-Kuan Ou via lttng-dev wrote:
> Read-side critical section nesting is tracked in lower-order bits
> and grace-period phase number use a single high-order bit
> 

Thanks for the fix. Here is a comment below,

> Signed-off-by: Li-Kuan Ou <k777k777tw at gmail.com>
> ---
>   include/urcu/static/urcu-bp.h     | 4 ++--
>   include/urcu/static/urcu-mb.h     | 4 ++--
>   include/urcu/static/urcu-memb.h   | 4 ++--
>   include/urcu/static/urcu-signal.h | 4 ++--
>   4 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/include/urcu/static/urcu-bp.h b/include/urcu/static/urcu-bp.h
> index 8ba3830..c90c9f1 100644
> --- a/include/urcu/static/urcu-bp.h
> +++ b/include/urcu/static/urcu-bp.h
> @@ -137,8 +137,8 @@ static inline enum urcu_bp_state urcu_bp_reader_state(unsigned long *ctr)
>   
>   /*
>    * Helper for _urcu_bp_read_lock().  The format of urcu_bp_gp.ctr (as well as
> - * the per-thread rcu_reader.ctr) has the upper bits containing a count of
> - * _urcu_bp_read_lock() nesting, and a lower-order bit that contains either zero
> + * the per-thread rcu_reader.ctr) has the lower-order bits containing a count of
> + * _urcu_bp_read_lock() nesting, and a single high-order bit that contains either zero

I think it would be clearer to state:

Helper for _urcu_bp_read_lock().  The format of urcu_bp_gp.ctr (as well as
the per-thread rcu_reader.ctr) has the lower-order bits containing a count of
urcu_bp_read_lock() nesting, and a single high-order URCU_BP_GP_CTR_PHASE bit
that contains either zero or one.  The smp_mb_slave() ensures that the accesses
in urcu_bp_read_lock() happen before the subsequent read-side critical section.

(likewise for similar comments in other files).

Can you submit an updated patch please ?

Thanks,

Mathieu



>    * or URCU_BP_GP_CTR_PHASE.  The smp_mb_slave() ensures that the accesses in
>    * _urcu_bp_read_lock() happen before the subsequent read-side critical section.
>    */
> diff --git a/include/urcu/static/urcu-mb.h b/include/urcu/static/urcu-mb.h
> index b97e42a..218e2f3 100644
> --- a/include/urcu/static/urcu-mb.h
> +++ b/include/urcu/static/urcu-mb.h
> @@ -63,8 +63,8 @@ extern DECLARE_URCU_TLS(struct urcu_reader, urcu_mb_reader);
>   
>   /*
>    * Helper for _urcu_mb_read_lock().  The format of urcu_mb_gp.ctr (as well as
> - * the per-thread rcu_reader.ctr) has the upper bits containing a count of
> - * _urcu_mb_read_lock() nesting, and a lower-order bit that contains either zero
> + * the per-thread rcu_reader.ctr) has the lower-order bits containing a count of
> + * _urcu_mb_read_lock() nesting, and a single high-order bit that contains either zero
>    * or URCU_GP_CTR_PHASE.  The cmm_smp_mb() ensures that the accesses in
>    * _urcu_mb_read_lock() happen before the subsequent read-side critical section.
>    */
> diff --git a/include/urcu/static/urcu-memb.h b/include/urcu/static/urcu-memb.h
> index c8d102f..b923f73 100644
> --- a/include/urcu/static/urcu-memb.h
> +++ b/include/urcu/static/urcu-memb.h
> @@ -86,8 +86,8 @@ extern DECLARE_URCU_TLS(struct urcu_reader, urcu_memb_reader);
>   
>   /*
>    * Helper for _rcu_read_lock().  The format of urcu_memb_gp.ctr (as well as
> - * the per-thread rcu_reader.ctr) has the upper bits containing a count of
> - * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
> + * the per-thread rcu_reader.ctr) has the lower-order bits containing a count of
> + * _rcu_read_lock() nesting, and a single high-order bit that contains either zero
>    * or URCU_GP_CTR_PHASE.  The smp_mb_slave() ensures that the accesses in
>    * _rcu_read_lock() happen before the subsequent read-side critical section.
>    */
> diff --git a/include/urcu/static/urcu-signal.h b/include/urcu/static/urcu-signal.h
> index c7577d3..00588b8 100644
> --- a/include/urcu/static/urcu-signal.h
> +++ b/include/urcu/static/urcu-signal.h
> @@ -64,8 +64,8 @@ extern DECLARE_URCU_TLS(struct urcu_reader, urcu_signal_reader);
>   
>   /*
>    * Helper for _rcu_read_lock().  The format of urcu_signal_gp.ctr (as well as
> - * the per-thread rcu_reader.ctr) has the upper bits containing a count of
> - * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
> + * the per-thread rcu_reader.ctr) has the lower-order bits containing a count of
> + * _rcu_read_lock() nesting, and a single high-order bit that contains either zero
>    * or URCU_GP_CTR_PHASE.  The cmm_barrier() ensures that the accesses in
>    * _rcu_read_lock() happen before the subsequent read-side critical section.
>    */

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com