[PATCH] iommu/vt-d: Fix a IOMMU perfmon warning when CPU hotplug

[kan.liang@linux.intel.com]

From: Kan Liang <kan.liang@linux.intel.com>

A warning can be triggered when hotplug CPU 0.
 $ echo 0 > /sys/devices/system/cpu/cpu0/online

[11958.737635] ------------[ cut here ]------------
[11958.742882] Voluntary context switch within RCU read-side critical
section!
[11958.742891] WARNING: CPU: 0 PID: 19 at kernel/rcu/tree_plugin.h:318
rcu_note_context_switch+0x4f4/0x580
[11958.860095] RIP: 0010:rcu_note_context_switch+0x4f4/0x580
[11958.960360] Call Trace:
[11958.963161]  <TASK>
[11958.965565]  ? perf_event_update_userpage+0x104/0x150
[11958.971293]  __schedule+0x8d/0x960
[11958.975165]  ? perf_event_set_state.part.82+0x11/0x50
[11958.980891]  schedule+0x44/0xb0
[11958.984464]  schedule_timeout+0x226/0x310
[11958.989017]  ? __perf_event_disable+0x64/0x1a0
[11958.994054]  ? _raw_spin_unlock+0x14/0x30
[11958.998605]  wait_for_completion+0x94/0x130
[11959.003352]  __wait_rcu_gp+0x108/0x130
[11959.007616]  synchronize_rcu+0x67/0x70
[11959.011876]  ? invoke_rcu_core+0xb0/0xb0
[11959.016333]  ? __bpf_trace_rcu_stall_warning+0x10/0x10
[11959.022147]  perf_pmu_migrate_context+0x121/0x370
[11959.027478]  iommu_pmu_cpu_offline+0x6a/0xa0
[11959.032325]  ? iommu_pmu_del+0x1e0/0x1e0
[11959.036782]  cpuhp_invoke_callback+0x129/0x510
[11959.041825]  cpuhp_thread_fun+0x94/0x150
[11959.046283]  smpboot_thread_fn+0x183/0x220
[11959.050933]  ? sort_range+0x20/0x20
[11959.054902]  kthread+0xe6/0x110
[11959.058479]  ? kthread_complete_and_exit+0x20/0x20
[11959.063911]  ret_from_fork+0x1f/0x30
[11959.067982]  </TASK>
[11959.070489] ---[ end trace 0000000000000000 ]---

The synchronize_rcu() will be invoked in the perf_pmu_migrate_context(),
when migrating a PMU to a new CPU. However, the current for_each_iommu()
is within RCU read-side critical section.

Use the dmar_global_lock to replace the RCU read lock when going through
the drhd list.

Fixes: 46284c6ceb5e ("iommu/vt-d: Support cpumask for IOMMU perfmon")
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
 drivers/iommu/intel/perfmon.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/intel/perfmon.c b/drivers/iommu/intel/perfmon.c
index e17d9743a0d8..81c9554fb1dc 100644
--- a/drivers/iommu/intel/perfmon.c
+++ b/drivers/iommu/intel/perfmon.c
@@ -797,14 +797,14 @@ static int iommu_pmu_cpu_offline(unsigned int cpu)
 	else
 		target = -1;
 
-	rcu_read_lock();
+	down_write(&dmar_global_lock);
 
 	for_each_iommu(iommu, drhd) {
 		if (!iommu->pmu)
 			continue;
 		perf_pmu_migrate_context(&iommu->pmu->pmu, cpu, target);
 	}
-	rcu_read_unlock();
+	up_write(&dmar_global_lock);
 
 	return 0;
 }
-- 
2.35.1