2 files changed, 49 insertions, 5 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5207c28c2402..50d8cc37498b 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -275,6 +275,9 @@ enum {
 	 * - "tasks" is removed.  Everything should be at process
 	 *   granularity.  Use "cgroup.procs" instead.
 	 *
+	 * - "cgroup.procs" is not sorted.  pids will be unique unless they
+	 *   got recycled inbetween reads.
+	 *
 	 * - "release_agent" and "notify_on_release" are removed.
 	 *   Replacement notification mechanism will be implemented.
 	 *
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a2458031d851..f9f5fe3526ac 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3564,11 +3564,49 @@ after:
 	return dest;
 }
 
+/*
+ * The two pid files - task and cgroup.procs - guaranteed that the result
+ * is sorted, which forced this whole pidlist fiasco.  As pid order is
+ * different per namespace, each namespace needs differently sorted list,
+ * making it impossible to use, for example, single rbtree of member tasks
+ * sorted by task pointer.  As pidlists can be fairly large, allocating one
+ * per open file is dangerous, so cgroup had to implement shared pool of
+ * pidlists keyed by cgroup and namespace.
+ *
+ * All this extra complexity was caused by the original implementation
+ * committing to an entirely unnecessary property.  In the long term, we
+ * want to do away with it.  Explicitly scramble sort order if
+ * sane_behavior so that no such expectation exists in the new interface.
+ *
+ * Scrambling is done by swapping every two consecutive bits, which is
+ * non-identity one-to-one mapping which disturbs sort order sufficiently.
+ */
+static pid_t pid_fry(pid_t pid)
+{
+	unsigned a = pid & 0x55555555;
+	unsigned b = pid & 0xAAAAAAAA;
+
+	return (a << 1) | (b >> 1);
+}
+
+static pid_t cgroup_pid_fry(struct cgroup *cgrp, pid_t pid)
+{
+	if (cgroup_sane_behavior(cgrp))
+		return pid_fry(pid);
+	else
+		return pid;
+}
+
 static int cmppid(const void *a, const void *b)
 {
 	return *(pid_t *)a - *(pid_t *)b;
 }
 
+static int fried_cmppid(const void *a, const void *b)
+{
+	return pid_fry(*(pid_t *)a) - pid_fry(*(pid_t *)b);
+}
+
 static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
 						  enum cgroup_filetype type)
 {
@@ -3656,7 +3694,10 @@ static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
 	css_task_iter_end(&it);
 	length = n;
 	/* now sort & (if procs) strip out duplicates */
-	sort(array, length, sizeof(pid_t), cmppid, NULL);
+	if (cgroup_sane_behavior(cgrp))
+		sort(array, length, sizeof(pid_t), fried_cmppid, NULL);
+	else
+		sort(array, length, sizeof(pid_t), cmppid, NULL);
 	if (type == CGROUP_FILE_PROCS)
 		length = pidlist_uniq(array, length);
 
@@ -3777,10 +3818,10 @@ static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
 
 		while (index < end) {
 			int mid = (index + end) / 2;
-			if (l->list[mid] == pid) {
+			if (cgroup_pid_fry(cgrp, l->list[mid]) == pid) {
 				index = mid;
 				break;
-			} else if (l->list[mid] <= pid)
+			} else if (cgroup_pid_fry(cgrp, l->list[mid]) <= pid)
 				index = mid + 1;
 			else
 				end = mid;
@@ -3791,7 +3832,7 @@ static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
 		return NULL;
 	/* Update the abstract position to be the actual pid that we found */
 	iter = l->list + index;
-	*pos = *iter;
+	*pos = cgroup_pid_fry(cgrp, *iter);
 	return iter;
 }
 
@@ -3820,7 +3861,7 @@ static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
 	if (p >= end) {
 		return NULL;
 	} else {
-		*pos = *p;
+		*pos = cgroup_pid_fry(of->cgrp, *p);
 		return p;
 	}
 }