Commit 24a57aff authored Apr 19, 2022 by Zqiang Committed by Paul E. McKenney Jul 19, 2022

rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order



In case of failure to spawn either rcuog or rcuo[p] kthreads for a given
rdp, rcu_nocb_rdp_deoffload() needs to be called with the hotplug
lock and the barrier_mutex held. However cpus write lock is already held
while calling rcutree_prepare_cpu(). It's not possible to call
rcu_nocb_rdp_deoffload() from there with just locking the barrier_mutex
or this would result in a locking inversion against
rcu_nocb_cpu_deoffload() which holds both locks in the reverse order.

Simply solve this with inverting the locking order inside
rcu_nocb_cpu_[de]offload(). This will be a pre-requisite to toggle NOCB
states toward cpusets anyway.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>

parent 1598f4a4

kernel/rcu/tree_nocb.h

+4 −4

Original line number	Diff line number	Diff line
		@@ -1055,8 +1055,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
		int ret = 0;

		mutex_lock(&rcu_state.barrier_mutex);
		cpus_read_lock();
		mutex_lock(&rcu_state.barrier_mutex);
		if (rcu_rdp_is_offloaded(rdp)) {
		if (cpu_online(cpu)) {
		ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
		@@ -1067,8 +1067,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
		ret = -EINVAL;
		}
		}
		cpus_read_unlock();
		mutex_unlock(&rcu_state.barrier_mutex);
		cpus_read_unlock();

		return ret;
		}
		@@ -1134,8 +1134,8 @@ int rcu_nocb_cpu_offload(int cpu)
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
		int ret = 0;

		mutex_lock(&rcu_state.barrier_mutex);
		cpus_read_lock();
		mutex_lock(&rcu_state.barrier_mutex);
		if (!rcu_rdp_is_offloaded(rdp)) {
		if (cpu_online(cpu)) {
		ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
		@@ -1146,8 +1146,8 @@ int rcu_nocb_cpu_offload(int cpu)
		ret = -EINVAL;
		}
		}
		cpus_read_unlock();
		mutex_unlock(&rcu_state.barrier_mutex);
		cpus_read_unlock();

		return ret;
		}