[PATCHv2 3/3] rcu: coordinate tick dependency during concurrent offlining

[Pingfan Liu <kernelfans@gmail.com>]

As Paul pointed out  "The tick_dep_clear() is SMP-safe because it uses
atomic operations, but the problem is that if there are multiple
nohz_full CPUs going offline concurrently, the first CPU to invoke
rcutree_dead_cpu() will turn the tick off.  This might require an
atomically manipulated counter to mediate the calls to
rcutree_dead_cpu(). "

This patch introduces a new member ->dying to rcu_node, which reflects
the number of concurrent offlining cpu. TICK_DEP_BIT_RCU is set by
the first entrance and cleared by the last.

Note: now, tick_dep_set() is put under the rnp->lock, but since it takes
no lock, no extra locking order is introduced.

Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: rcu@vger.kernel.org
---
 kernel/rcu/tree.c | 19 ++++++++++++++-----
 kernel/rcu/tree.h |  1 +
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8a829b64f5b2..f8bd0fc5fd2f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2164,13 +2164,19 @@ int rcutree_dead_cpu(unsigned int cpu)
 {
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
+	unsigned long flags;
+	u8 dying;
 
 	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
 		return 0;
 
 	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
-	// Stop-machine done, so allow nohz_full to disable tick.
-	tick_dep_clear(TICK_DEP_BIT_RCU);
+	raw_spin_lock_irqsave_rcu_node(rnp, flags);
+	dying = --rnp->dying;
+	if (!dying)
+		// Stop-machine done, so allow nohz_full to disable tick.
+		tick_dep_clear(TICK_DEP_BIT_RCU);
+	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	return 0;
 }
 
@@ -4020,17 +4026,20 @@ int rcutree_offline_cpu(unsigned int cpu)
 	unsigned long flags;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
+	u8 dying;
 
 	rdp = per_cpu_ptr(&rcu_data, cpu);
 	rnp = rdp->mynode;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rnp->ffmask &= ~rdp->grpmask;
+	/* Let rcutree_dead_cpu() know a new offlining. */
+	dying = rnp->dying++;
+	if (!dying)
+		// nohz_full CPUs need the tick for stop-machine to work quickly
+		tick_dep_set(TICK_DEP_BIT_RCU);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
 	rcutree_affinity_setting(cpu, cpu);
-
-	// nohz_full CPUs need the tick for stop-machine to work quickly
-	tick_dep_set(TICK_DEP_BIT_RCU);
 	return 0;
 }
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d4a97e40ea9c..b508a12ac953 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -81,6 +81,7 @@ struct rcu_node {
 	int	grphi;		/* highest-numbered CPU here. */
 	u8	grpnum;		/* group number for next level up. */
 	u8	level;		/* root is at level 0. */
+	u8	dying;		/* num of concurrent rdp offlining */
 	bool	wait_blkd_tasks;/* Necessary to wait for blocked tasks to */
 				/*  exit RCU read-side critical sections */
 				/*  before propagating offline up the */
-- 
2.31.1