From: Andrew Morton <akpm@linux-foundation.org>

put this back - hotplug-cpu-migrate-a-task-within-its-cpuset.patch uses it.

Cc: Andrea Arcangeli <andrea@suse.de>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/cpuset.h |    6 ++++++
 kernel/cpuset.c        |   27 +++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff -puN include/linux/cpuset.h~oom-do-not-take-callback_mutex-fix include/linux/cpuset.h
--- a/include/linux/cpuset.h~oom-do-not-take-callback_mutex-fix
+++ a/include/linux/cpuset.h
@@ -59,6 +59,9 @@ extern void __cpuset_memory_pressure_bum
 extern const struct file_operations proc_cpuset_operations;
 extern char *cpuset_task_status_allowed(struct task_struct *task, char *buffer);
 
+extern void cpuset_lock(void);
+extern void cpuset_unlock(void);
+
 extern int cpuset_mem_spread_node(void);
 
 static inline int cpuset_do_page_mem_spread(void)
@@ -125,6 +128,9 @@ static inline char *cpuset_task_status_a
 	return buffer;
 }
 
+static inline void cpuset_lock(void) {}
+static inline void cpuset_unlock(void) {}
+
 static inline int cpuset_mem_spread_node(void)
 {
 	return 0;
diff -puN kernel/cpuset.c~oom-do-not-take-callback_mutex-fix kernel/cpuset.c
--- a/kernel/cpuset.c~oom-do-not-take-callback_mutex-fix
+++ a/kernel/cpuset.c
@@ -2521,6 +2521,33 @@ int __cpuset_zone_allowed_hardwall(struc
 }
 
 /**
+ * cpuset_lock - lock out any changes to cpuset structures
+ *
+ * The out of memory (oom) code needs to mutex_lock cpusets
+ * from being changed while it scans the tasklist looking for a
+ * task in an overlapping cpuset.  Expose callback_mutex via this
+ * cpuset_lock() routine, so the oom code can lock it, before
+ * locking the task list.  The tasklist_lock is a spinlock, so
+ * must be taken inside callback_mutex.
+ */
+
+void cpuset_lock(void)
+{
+	mutex_lock(&callback_mutex);
+}
+
+/**
+ * cpuset_unlock - release lock on cpuset changes
+ *
+ * Undo the lock taken in a previous cpuset_lock() call.
+ */
+
+void cpuset_unlock(void)
+{
+	mutex_unlock(&callback_mutex);
+}
+
+/**
  * cpuset_mem_spread_node() - On which node to begin search for a page
  *
  * If a task is marked PF_SPREAD_PAGE or PF_SPREAD_SLAB (as for
_