iommu/vt-d: Use rcu_lock in get_resv_regions

Commit 5f64ce5 ("iommu/vt-d: Duplicate iommu_resv_region objects
per device list") converted rcu_lock in get_resv_regions to
dmar_global_lock to allow sleeping in iommu_alloc_resv_region(). This
introduced possible recursive locking if get_resv_regions is called from
within a section where intel_iommu_init() already holds dmar_global_lock.

Especially, after commit 57365a0 ("iommu: Move bus setup to IOMMU
device registration"), below lockdep splats could always be seen.

 ============================================
 WARNING: possible recursive locking detected
 6.0.0-rc4+ #325 Tainted: G          I
 --------------------------------------------
 swapper/0/1 is trying to acquire lock:
 ffffffffa8a18c90 (dmar_global_lock){++++}-{3:3}, at:
 intel_iommu_get_resv_regions+0x25/0x270

 but task is already holding lock:
 ffffffffa8a18c90 (dmar_global_lock){++++}-{3:3}, at:
 intel_iommu_init+0x36d/0x6ea

 ...

 Call Trace:
  <TASK>
  dump_stack_lvl+0x48/0x5f
  __lock_acquire.cold.73+0xad/0x2bb
  lock_acquire+0xc2/0x2e0
  ? intel_iommu_get_resv_regions+0x25/0x270
  ? lock_is_held_type+0x9d/0x110
  down_read+0x42/0x150
  ? intel_iommu_get_resv_regions+0x25/0x270
  intel_iommu_get_resv_regions+0x25/0x270
  iommu_create_device_direct_mappings.isra.28+0x8d/0x1c0
  ? iommu_get_dma_cookie+0x6d/0x90
  bus_iommu_probe+0x19f/0x2e0
  iommu_device_register+0xd4/0x130
  intel_iommu_init+0x3e1/0x6ea
  ? iommu_setup+0x289/0x289
  ? rdinit_setup+0x34/0x34
  pci_iommu_init+0x12/0x3a
  do_one_initcall+0x65/0x320
  ? rdinit_setup+0x34/0x34
  ? rcu_read_lock_sched_held+0x5a/0x80
  kernel_init_freeable+0x28a/0x2f3
  ? rest_init+0x1b0/0x1b0
  kernel_init+0x1a/0x130
  ret_from_fork+0x1f/0x30
  </TASK>

This rolls back dmar_global_lock to rcu_lock in get_resv_regions to avoid
the lockdep splat.

Fixes: 57365a0 ("iommu: Move bus setup to IOMMU device registration")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20220927053109.4053662-3-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel/iommu.c

@@ -4534,7 +4534,7 @@ static void intel_iommu_get_resv_regions(struct device *device,
 	struct device *i_dev;
 	int i;
 
-	down_read(&dmar_global_lock);
+	rcu_read_lock();
 	for_each_rmrr_units(rmrr) {
 		for_each_active_dev_scope(rmrr->devices, rmrr->devices_cnt,
 					  i, i_dev) {
@@ -4553,14 +4553,14 @@ static void intel_iommu_get_resv_regions(struct device *device,
 
 			resv = iommu_alloc_resv_region(rmrr->base_address,
 						       length, prot, type,
-						       GFP_KERNEL);
+						       GFP_ATOMIC);
 			if (!resv)
 				break;
 
 			list_add_tail(&resv->list, head);
 		}
 	}
-	up_read(&dmar_global_lock);
+	rcu_read_unlock();
 
 #ifdef CONFIG_INTEL_IOMMU_FLOPPY_WA
 	if (dev_is_pci(device)) {