linux-kernel - Re: [PATCH] rcu: simplify the calculation of rcu

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Date:   Fri, 17 Apr 2020 20:19:22 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     josh@...htriplett.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: simplify the calculation of rcu_state.ncpus

On Fri, Apr 17, 2020 at 09:39:51PM +0000, Wei Yang wrote:
> There is only 1 bit set in mask, which means the difference between
> oldmask and the new one would be at the position where the bit is set in
> mask.
> 
> Based on this knowledge, rcu_state.ncpus could be calculated by checking
> whether mask is already set in oldmask.

Nice!!!  Good eyes!

> BTW, the comment at the last of this line is mysterious. Not sure it
> could be removed or not.

The "^^^" in that comment says to look at the comment on the preceding
line.  Memory-ordering functions like smp_store_release() are supposed
to have comments indicating what they are ordering.  ;-)

Could you please do the following things and resubmit?

1.	Forward-port to -rcu branch dev?  This tree lives here:
	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

2.	Given that oldmask is used only to test to see if a new bit
	was set, why not just replace oldmask with a bool variable
	that is set to "!(rnp->expmaskinitnext & mask)" before the
	bit is ORed into rnp->expmaskinitnext?

3.	Put the comment inside the "if" statement with the
	smp_store_release().

4.	In -rcu, you will find a ASSERT_EXCLUSIVE_WRITER() statement
	that should also be placed inside the "if" statement with
	the smp_store_release().

							Thanx, Paul

> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>  kernel/rcu/tree.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d91c9156fab2..f0d9251fa663 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3364,7 +3364,6 @@ void rcu_cpu_starting(unsigned int cpu)
>  {
>  	unsigned long flags;
>  	unsigned long mask;
> -	int nbits;
>  	unsigned long oldmask;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp;
> @@ -3381,10 +3380,9 @@ void rcu_cpu_starting(unsigned int cpu)
>  	rnp->qsmaskinitnext |= mask;
>  	oldmask = rnp->expmaskinitnext;
>  	rnp->expmaskinitnext |= mask;
> -	oldmask ^= rnp->expmaskinitnext;
> -	nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
>  	/* Allow lockless access for expedited grace periods. */
> -	smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + nbits); /* ^^^ */
> +	if (!(oldmask & mask))
> +		smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + 1); /* ^^^ */
>  	rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
>  	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
>  	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> -- 
> 2.23.0
>