[PATCH] ACPI / DMAR: Avoid passing NULL to acpi_put_table()

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Linus reported that commit 174cc7187e6f "ACPICA: Tables: Back port
acpi_get_table_with_size() and early_acpi_os_unmap_memory() from
Linux kernel" added a new warning on his desktop system:

 ACPI Warning: Table ffffffff9fe6c0a0, Validation count is zero before decrement

which turns out to come from the acpi_put_table() in
detect_intel_iommu().

This happens if the DMAR table is not present in which case NULL is
passed to acpi_put_table() which doesn't check against that and
attempts to handle it regardless.

For this reason, check the pointer passed to acpi_put_table()
before invoking it.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org> 
Fixes: 6b11d1d67713 ("ACPI / osl: Remove acpi_get_table_with_size()/early_acpi_os_unmap_memory() users")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/iommu/dmar.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-pm/drivers/iommu/dmar.c
===================================================================
--- linux-pm.orig/drivers/iommu/dmar.c
+++ linux-pm/drivers/iommu/dmar.c
@@ -903,8 +903,10 @@ int __init detect_intel_iommu(void)
 		x86_init.iommu.iommu_init = intel_iommu_init;
 #endif
 
-	acpi_put_table(dmar_tbl);
-	dmar_tbl = NULL;
+	if (dmar_tbl) {
+		acpi_put_table(dmar_tbl);
+		dmar_tbl = NULL;
+	}
 	up_write(&dmar_global_lock);
 
 	return ret ? 1 : -ENODEV;