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@xxxxxxxxx>
> ---
> 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
>
