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rcu: Simplify the calculation of rcu_state.ncpus

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There is only 1 bit set in mask, which means that the only difference
between oldmask and the new one will be at the position where the bit is
set in mask.  This commit therefore updates rcu_state.ncpus by checking
whether the bit in mask is already set in rnp->expmaskinitnext.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Wei Yang 2020-04-19 21:57:15 +00:00 committed by Paul E. McKenney
parent 0a3b3c253a
commit abfce04148
1 changed files with 3 additions and 6 deletions
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9
kernel/rcu/tree.c
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@ -3842,10 +3842,9 @@ 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;
bool newcpu;
if (per_cpu(rcu_cpu_started, cpu))
return;
@ -3857,12 +3856,10 @@ void rcu_cpu_starting(unsigned int cpu)
mask = rdp->grpmask;
raw_spin_lock_irqsave_rcu_node(rnp, flags);
WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
oldmask = rnp->expmaskinitnext;
newcpu = !(rnp->expmaskinitnext & mask);
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); /* ^^^ */
smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + newcpu); /* ^^^ */
ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);