Re: [PATCH] rcu: Further comment and explain the state space of GP sequences

[Joel Fernandes]

On Thu, Jan 19, 2023 at 2:15 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> On Thu, Jan 19, 2023 at 03:11:35PM +0100, Frederic Weisbecker wrote:
> > The state space of the GP sequence number isn't documented and the
> > definitions of its special values are scattered. Try to gather some
> > common knowledge near the GP seq headers.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > ---
> >  kernel/rcu/rcu.h | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index 115616ac3bfa..fb95de039596 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -14,6 +14,39 @@
> >
> >  /*
> >   * Grace-period counter management.
> > + *
> > + * The two lowest significant bits gather the control flags.
> > + * The higher bits form the RCU sequence counter.
> > + *
> > + * About the control flags, a common value of 0 means that no GP is in progress.
> > + * A value of 1 means that a grace period has started and is in progress. When
> > + * the grace period completes, the control flags are reset to 0 and the sequence
> > + * counter is incremented.
> > + *
> > + * However some specific RCU usages make use of custom values.
> > + *
> > + * SRCU special control values:
> > + *
> > + *   SRCU_SNP_INIT_SEQ       :       Invalid/init value set when SRCU node
> > + *                                                   is initialized.
> > + *
> > + *   SRCU_STATE_IDLE         :       No SRCU gp is in progress
> > + *
> > + *   SRCU_STATE_SCAN1        :       State set by rcu_seq_start(). Indicates
> > + *                                                           we are scanning the inactive readers
> > + *                                                           index.

The term "inactive reader" is confusing. The readers can very much be
active during scans. During a scan stage, there might be a reader on
any of the 2 indexes that can be right in the middle of their critical
section (and we don't know which index because they could have got
preempted, right after sampling idx). Maybe "inactive slot" is a
better term? And define "inactive slot" as the slot which is no longer
going to be sampled by new readers.

> > + *
> > + *           SRCU_STATE_SCAN2        :       State set manually via rcu_seq_set_state()
> > + *                                                           Indicates we are flipping the readers
> > + *                                                           index and then scanning the newly inactive
> > + *                                                           readers index.
> > + *
> > + * RCU polled GP special control value:
> > + *
> > + *   RCU_GET_STATE_COMPLETED :       State value indicating that a polled GP
> > + *                                                           has completed. It's an absolute value
> > + *                                                           covering both the state and the counter of
> > + *                                                           the GP sequence.

This part I still have to learn (polling) so I'll go do that. But
otherwise your patch LGTM and thanks for doing it.

 - Joel

> >   */
> >
> >  #define RCU_SEQ_CTR_SHIFT    2
> > --
> > 2.34.1
> >
>
> Ok perhaps this one got the tabs right:
>
> ---
> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Date: Thu, 19 Jan 2023 14:29:34 +0100
> Subject: [PATCH v2] rcu: Further comment and explain the state space of GP
>  sequences
>
> The state space of the GP sequence number isn't documented and the
> definitions of its special values are scattered. Try to gather some
> common knowledge near the GP seq headers.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  kernel/rcu/rcu.h | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
>
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 115616ac3bfa..fb95de039596 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -14,6 +14,39 @@
>
>  /*
>   * Grace-period counter management.
> + *
> + * The two lowest significant bits gather the control flags.
> + * The higher bits form the RCU sequence counter.
> + *
> + * About the control flags, a common value of 0 means that no GP is in progress.
> + * A value of 1 means that a grace period has started and is in progress. When
> + * the grace period completes, the control flags are reset to 0 and the sequence
> + * counter is incremented.
> + *
> + * However some specific RCU usages make use of custom values.
> + *
> + * SRCU special control values:
> + *
> + *     SRCU_SNP_INIT_SEQ       :       Invalid/init value set when SRCU node
> + *                                     is initialized.
> + *
> + *     SRCU_STATE_IDLE         :       No SRCU gp is in progress
> + *
> + *     SRCU_STATE_SCAN1        :       State set by rcu_seq_start(). Indicates
> + *                                     we are scanning the inactive readers
> + *                                     index.
> + *
> + *     SRCU_STATE_SCAN2        :       State set manually via rcu_seq_set_state()
> + *                                     Indicates we are flipping the readers
> + *                                     index and then scanning the newly inactive
> + *                                     readers index.
> + *
> + * RCU polled GP special control value:
> + *
> + *     RCU_GET_STATE_COMPLETED :       State value indicating that a polled GP
> + *                                     has completed. It's an absolute value
> + *                                     covering both the state and the counter of
> + *                                     the GP sequence.
>   */
>
>  #define RCU_SEQ_CTR_SHIFT      2
> --
> 2.34.1
>