cgroups: ensure correct concurrent opening/reading of pidlists across pid namespaces

Previously there was the problem in which two processes from different pid
namespaces reading the tasks or procs file could result in one process
seeing results from the other's namespace.  Rather than one pidlist for
each file in a cgroup, we now keep a list of pidlists keyed by namespace
and file type (tasks versus procs) in which entries are placed on demand.
Each pidlist has its own lock, and that the pidlists themselves are passed
around in the seq_file's private pointer means we don't have to touch the
cgroup or its master list except when creating and destroying entries.

Signed-off-by: Ben Blum <bblum@google.com>
Signed-off-by: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 29 insertions, 5 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2357733a0a80..88e863460726 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -141,15 +141,36 @@ enum {
         CGRP_WAIT_ON_RMDIR,
 };
 
+/* which pidlist file are we talking about? */
+enum cgroup_filetype {
+        CGROUP_FILE_PROCS,
+        CGROUP_FILE_TASKS,
+};
+
+/*
+ * A pidlist is a list of pids that virtually represents the contents of one
+ * of the cgroup files ("procs" or "tasks"). We keep a list of such pidlists,
+ * a pair (one each for procs, tasks) for each pid namespace that's relevant
+ * to the cgroup.
+ */
 struct cgroup_pidlist {
-        /* protects the other fields */
-        struct rw_semaphore mutex;
+        /*
+         * used to find which pidlist is wanted. doesn't change as long as
+         * this particular list stays in the list.
+         */
+        struct { enum cgroup_filetype type; struct pid_namespace *ns; } key;
         /* array of xids */
         pid_t *list;
         /* how many elements the above list has */
         int length;
         /* how many files are using the current array */
         int use_count;
+        /* each of these stored in a list by its cgroup */
+        struct list_head links;
+        /* pointer to the cgroup we belong to, for list removal purposes */
+        struct cgroup *owner;
+        /* protects the other fields */
+        struct rw_semaphore mutex;
 };
 
 struct cgroup {
@@ -190,9 +211,12 @@ struct cgroup {
          */
         struct list_head release_list;
 
-        /* we will have two separate pidlists, one for pids (the tasks file)
-         * and one for tgids (the procs file). */
-        struct cgroup_pidlist tasks, procs;
+        /*
+         * list of pidlists, up to two for each namespace (one for procs, one
+         * for tasks); created on demand.
+         */
+        struct list_head pidlists;
+        struct mutex pidlist_mutex;
 
         /* For RCU-protected deletion */
         struct rcu_head rcu_head;