[PATCH] acpi: fix NULL pointer dereference in acpi_ex_release_mutex()

[Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>]

From: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Subject: [PATCH] acpi: fix NULL pointer dereference in acpi_ex_release_mutex()

>From Dan's list:

drivers/acpi/acpica/exmutex.c +397 acpi_ex_release_mutex(40) warning: variable derefenced before check 'walk_state->thread'

Reorder the code to make it check for walk_state->thread existence before
accessing walk_state->thread->thread_id and fix the comment while at it.

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: corbet@lwn.net
Cc: eteo@redhat.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
---
2.6.31 material

 drivers/acpi/acpica/exmutex.c |   17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

Index: b/drivers/acpi/acpica/exmutex.c
===================================================================
--- a/drivers/acpi/acpica/exmutex.c
+++ b/drivers/acpi/acpica/exmutex.c
@@ -375,6 +375,14 @@ acpi_ex_release_mutex(union acpi_operand
 		return_ACPI_STATUS(AE_AML_MUTEX_NOT_ACQUIRED);
 	}
 
+	/* must have a valid thread */
+	if (!walk_state->thread) {
+		ACPI_ERROR((AE_INFO,
+			    "Cannot release Mutex [%4.4s], null thread info",
+			    acpi_ut_get_node_name(obj_desc->mutex.node)));
+		return_ACPI_STATUS(AE_AML_INTERNAL);
+	}
+
 	/*
 	 * The Mutex is owned, but this thread must be the owner.
 	 * Special case for Global Lock, any thread can release
@@ -392,15 +400,6 @@ acpi_ex_release_mutex(union acpi_operand
 		return_ACPI_STATUS(AE_AML_NOT_OWNER);
 	}
 
-	/* Must have a valid thread ID */
-
-	if (!walk_state->thread) {
-		ACPI_ERROR((AE_INFO,
-			    "Cannot release Mutex [%4.4s], null thread info",
-			    acpi_ut_get_node_name(obj_desc->mutex.node)));
-		return_ACPI_STATUS(AE_AML_INTERNAL);
-	}
-
 	/*
 	 * The sync level of the mutex must be equal to the current sync level. In
 	 * other words, the current level means that at least one mutex at that