+static struct task_struct *cpuset_sem_owner;
+static int cpuset_sem_depth;
+
+/*
+ * The global cpuset semaphore cpuset_sem can be needed by the
+ * memory allocator to update a tasks mems_allowed (see the calls
+ * to cpuset_update_current_mems_allowed()) or to walk up the
+ * cpuset hierarchy to find a mem_exclusive cpuset see the calls
+ * to cpuset_excl_nodes_overlap()).
+ *
+ * But if the memory allocation is being done by cpuset.c code, it
+ * usually already holds cpuset_sem. Double tripping on a kernel
+ * semaphore deadlocks the current task, and any other task that
+ * subsequently tries to obtain the lock.
+ *
+ * Run all up's and down's on cpuset_sem through the following
+ * wrappers, which will detect this nested locking, and avoid
+ * deadlocking.
+ */
+
+static inline void cpuset_down(struct semaphore *psem)
+{
+ if (cpuset_sem_owner != current) {
+ down(psem);
+ cpuset_sem_owner = current;
+ }
+ cpuset_sem_depth++;
+}
+
+static inline void cpuset_up(struct semaphore *psem)
+{
+ if (--cpuset_sem_depth == 0) {
+ cpuset_sem_owner = NULL;
+ up(psem);
+ }
+}