linux-kernel - Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq

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Message-ID: <20180514050536.GB80415@joelaf.mtv.corp.google.com>
Date:   Sun, 13 May 2018 22:05:36 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com
Subject: Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap
 works

On Sun, May 13, 2018 at 08:47:24PM -0700, Randy Dunlap wrote:
> On 05/13/2018 08:15 PM, Joel Fernandes (Google) wrote:
> > rcu_seq_snap may be tricky for someone looking at it for the first time.
> > Lets document how it works with an example to make it easier.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/rcu.h | 24 +++++++++++++++++++++++-
> >  1 file changed, 23 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index 003671825d62..fc3170914ac7 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -91,7 +91,29 @@ static inline void rcu_seq_end(unsigned long *sp)
> >  	WRITE_ONCE(*sp, rcu_seq_endval(sp));
> >  }
> >  
> > -/* Take a snapshot of the update side's sequence number. */
> > +/*
> > + * Take a snapshot of the update side's sequence number.
> > + *
> > + * This function predicts what the grace period number will be the next
> > + * time an RCU callback will be executed, given the current grace period's
> > + * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is
> > + * already in progress.
> > + *
> > + * We do this with a single addition and masking.
> > + * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit (LSB) of
> > + * the seq is used to track if a GP is in progress or not, its sufficient if we
> 
>                                                               it's

Pardon my english. Fixed, thanks,

- Joel