Messages in this thread | | | Date | Mon, 21 May 2018 16:25:38 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq_needed |
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On Sun, May 20, 2018 at 09:42:20PM -0700, Joel Fernandes wrote: > We acquire gp_seq_needed locklessly. To be safe, lets do the unlocking > after the access.
Actually, no, we hold rnp_start's ->lock throughout. And this CPU (or in the case of no-CBs CPUs, this task) is in charge of rdp->gp_seq_needed, so nothing else is accessing it. Or at least that is the intent. ;-)
One exception is CPU hotplug, but in that case, only the CPU doing the hotplugging is allowed to touch rdp->gp_seq_needed and even then only while the incoming/outgoing CPU is inactive.
Thanx, Paul
> Signed-off-by: Joel Fernandes <joel@joelfernandes.org> > --- > kernel/rcu/tree.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 879c67a31116..efbd21b2a1a6 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1603,13 +1603,13 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp, > trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq")); > ret = true; /* Caller must wake GP kthread. */ > unlock_out: > - if (rnp != rnp_start) > - raw_spin_unlock_rcu_node(rnp); > /* Push furthest requested GP to leaf node and rcu_data structure. */ > if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) { > rnp_start->gp_seq_needed = gp_seq_req; > rdp->gp_seq_needed = gp_seq_req; > } > + if (rnp != rnp_start) > + raw_spin_unlock_rcu_node(rnp); > return ret; > } > > -- > 2.17.0.441.gb46fe60e1d-goog >
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