cgroups: convert tasks file to use a seq_file with shared pid array

Rather than pre-generating the entire text for the "tasks" file each
time the file is opened, we instead just generate/update the array of
process ids and use a seq_file to report these to userspace.  All open
file handles on the same "tasks" file can share a pid array, which may
be updated any time that no thread is actively reading the array.  By
sharing the array, the potential for userspace to DoS the system by
opening many handles on the same "tasks" file is removed.

[Based on a patch by Lai Jiangshan, extended to use seq_file]

Signed-off-by: Paul Menage <menage@google.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 7166023e07d2..8ab91880a0ad 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -14,6 +14,7 @@
 #include <linux/rcupdate.h>
 #include <linux/cgroupstats.h>
 #include <linux/prio_heap.h>
+#include <linux/rwsem.h>
 
 #ifdef CONFIG_CGROUPS
 
@@ -136,6 +137,15 @@ struct cgroup {
          * release_list_lock
          */
         struct list_head release_list;
+
+        /* pids_mutex protects the fields below */
+        struct rw_semaphore pids_mutex;
+        /* Array of process ids in the cgroup */
+        pid_t *tasks_pids;
+        /* How many files are using the current tasks_pids array */
+        int pids_use_count;
+        /* Length of the current tasks_pids array */
+        int pids_length;
 };
 
 /* A css_set is a structure holding pointers to a set of