From 75182a4eaaf8b697f66d68ad039f021f461dd2a4 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 2 Mar 2022 11:01:37 -0800 Subject: [PATCH] rcu: Add comments to final rcu_gp_cleanup() "if" statement The final "if" statement in rcu_gp_cleanup() has proven to be rather confusing, straightforward though it might have seemed when initially written. This commit therefore adds comments to its "then" and "else" clauses to at least provide a more elevated form of confusion. Reported-by: Boqun Feng Reported-by: Frederic Weisbecker Reported-by: Neeraj Upadhyay Reported-by: Uladzislau Rezki Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 25 ++++++++++++++++++++----- 1 file changed, 20 insertions(+), 5 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a5ea67454640..29669070348e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2098,14 +2098,29 @@ static noinline void rcu_gp_cleanup(void) /* Advance CBs to reduce false positives below. */ offloaded = rcu_rdp_is_offloaded(rdp); if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) { + + // We get here if a grace period was needed (“needgp”) + // and the above call to rcu_accelerate_cbs() did not set + // the RCU_GP_FLAG_INIT bit in ->gp_state (which records + // the need for another grace period).  The purpose + // of the “offloaded” check is to avoid invoking + // rcu_accelerate_cbs() on an offloaded CPU because we do not + // hold the ->nocb_lock needed to safely access an offloaded + // ->cblist.  We do not want to acquire that lock because + // it can be heavily contended during callback floods. + WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT); WRITE_ONCE(rcu_state.gp_req_activity, jiffies); - trace_rcu_grace_period(rcu_state.name, - rcu_state.gp_seq, - TPS("newreq")); + trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("newreq")); } else { - WRITE_ONCE(rcu_state.gp_flags, - rcu_state.gp_flags & RCU_GP_FLAG_INIT); + + // We get here either if there is no need for an + // additional grace period or if rcu_accelerate_cbs() has + // already set the RCU_GP_FLAG_INIT bit in ->gp_flags.  + // So all we need to do is to clear all of the other + // ->gp_flags bits. + + WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags & RCU_GP_FLAG_INIT); } raw_spin_unlock_irq_rcu_node(rnp);