cgroups: revert "cgroups: fix pid namespace bug"

The following series adds a "cgroup.procs" file to each cgroup that
reports unique tgids rather than pids, and allows all threads in a
threadgroup to be atomically moved to a new cgroup.

The subsystem "attach" interface is modified to support attaching whole
threadgroups at a time, which could introduce potential problems if any
subsystem were to need to access the old cgroup of every thread being
moved.  The attach interface may need to be revised if this becomes the
case.

Also added is functionality for read/write locking all CLONE_THREAD
fork()ing within a threadgroup, by means of an rwsem that lives in the
sighand_struct, for per-threadgroup-ness and also for sharing a cacheline
with the sighand's atomic count.  This scheme should introduce no extra
overhead in the fork path when there's no contention.

The final patch reveals potential for a race when forking before a
subsystem's attach function is called - one potential solution in case any
subsystem has this problem is to hang on to the group's fork mutex through
the attach() calls, though no subsystem yet demonstrates need for an
extended critical section.

This patch:

Revert

commit 096b7fe012d66ed55e98bc8022405ede0cc80e96
Author:     Li Zefan <lizf@cn.fujitsu.com>
AuthorDate: Wed Jul 29 15:04:04 2009 -0700
Commit:     Linus Torvalds <torvalds@linux-foundation.org>
CommitDate: Wed Jul 29 19:10:35 2009 -0700

    cgroups: fix pid namespace bug

This is in preparation for some clashing cgroups changes that subsume the
original commit's functionaliy.

The original commit fixed a pid namespace bug which Ben Blum fixed
independently (in the same way, but with different code) as part of a
series of patches.  I played around with trying to reconcile Ben's patch
series with Li's patch, but concluded that it was simpler to just revert
Li's, given that Ben's patch series contained essentially the same fix.

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, 7 insertions, 4 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 90bba9e62286..c833d6f23672 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -179,11 +179,14 @@ struct cgroup {
          */
         struct list_head release_list;
 
-        /* pids_mutex protects pids_list and cached pid arrays. */
+        /* pids_mutex protects the fields below */
         struct rw_semaphore pids_mutex;
-
-        /* Linked list of struct cgroup_pids */
-        struct list_head pids_list;
+        /* 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;
 
         /* For RCU-protected deletion */
         struct rcu_head rcu_head;