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

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Date:   Fri, 20 Jan 2023 01:47:16 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        quic_neeraju@...cinc.com, Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH] rcu: Further comment and explain the state space of GP
 sequences

On Thu, Jan 19, 2023 at 03:15:40PM +0100, Frederic Weisbecker 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@...nel.org>
> > ---
> >  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
> > 
> 
> Ok perhaps this one got the tabs right:
> 
> ---
> From: Frederic Weisbecker <frederic@...nel.org>
> 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@...nel.org>
> ---
>  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.
>   */

Other than the minor comments in the other thread:

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel


>  
>  #define RCU_SEQ_CTR_SHIFT	2
> -- 
> 2.34.1
>