iommu/vt-d: Fix suspicious RCU usage in probe_acpi_namespace_devices()

The drhd and device scope list should be iterated with the
iommu global lock held. Otherwise, a suspicious RCU usage
message will be displayed.

[    3.695886] =============================
[    3.695917] WARNING: suspicious RCU usage
[    3.695950] 5.2.0-rc2+ #2467 Not tainted
[    3.695981] -----------------------------
[    3.696014] drivers/iommu/intel-iommu.c:4569 suspicious rcu_dereference_check() usage!
[    3.696069]
               other info that might help us debug this:

[    3.696126]
               rcu_scheduler_active = 2, debug_locks = 1
[    3.696173] no locks held by swapper/0/1.
[    3.696204]
               stack backtrace:
[    3.696241] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.2.0-rc2+ #2467
[    3.696370] Call Trace:
[    3.696404]  dump_stack+0x85/0xcb
[    3.696441]  intel_iommu_init+0x128c/0x13ce
[    3.696478]  ? kmem_cache_free+0x16b/0x2c0
[    3.696516]  ? __fput+0x14b/0x270
[    3.696550]  ? __call_rcu+0xb7/0x300
[    3.696583]  ? get_max_files+0x10/0x10
[    3.696631]  ? set_debug_rodata+0x11/0x11
[    3.696668]  ? e820__memblock_setup+0x60/0x60
[    3.696704]  ? pci_iommu_init+0x16/0x3f
[    3.696737]  ? set_debug_rodata+0x11/0x11
[    3.696770]  pci_iommu_init+0x16/0x3f
[    3.696805]  do_one_initcall+0x5d/0x2e4
[    3.696844]  ? set_debug_rodata+0x11/0x11
[    3.696880]  ? rcu_read_lock_sched_held+0x6b/0x80
[    3.696924]  kernel_init_freeable+0x1f0/0x27c
[    3.696961]  ? rest_init+0x260/0x260
[    3.696997]  kernel_init+0xa/0x110
[    3.697028]  ret_from_fork+0x3a/0x50

Fixes: fa212a97 ("iommu/vt-d: Probe DMA-capable ACPI name space devices")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel-iommu.c

@@ -4821,8 +4821,10 @@ int __init intel_iommu_init(void)
 	cpuhp_setup_state(CPUHP_IOMMU_INTEL_DEAD, "iommu/intel:dead", NULL,
 			  intel_iommu_cpu_dead);
 
+	down_read(&dmar_global_lock);
 	if (probe_acpi_namespace_devices())
 		pr_warn("ACPI name space devices didn't probe correctly\n");
+	up_read(&dmar_global_lock);
 
 	/* Finally, we enable the DMA remapping hardware. */
 	for_each_iommu(iommu, drhd) {