mm: remove cgroup_mm_owner_callbacks

cgroup_mm_owner_callbacks() was brought in to support the memrlimit
controller, but sneaked into mainline ahead of it.  That controller has
now been shelved, and the mm_owner_changed() args were inadequate for it
anyway (they needed an mm pointer instead of a task pointer).

Remove the dead code, and restore mm_update_next_owner() locking to how it
was before: taking mmap_sem there does nothing for memcontrol.c, now the
only user of mm->owner.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 6 insertions, 43 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 87bb0258fd27..f221446aa02d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -116,7 +116,6 @@ static int root_count;
  * be called.
  */
 static int need_forkexit_callback __read_mostly;
-static int need_mm_owner_callback __read_mostly;
 
 /* convenient tests for these bits */
 inline int cgroup_is_removed(const struct cgroup *cgrp)
@@ -2539,7 +2538,6 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
 
         need_forkexit_callback |= ss->fork || ss->exit;
-        need_mm_owner_callback |= !!ss->mm_owner_changed;
 
         /* At system boot, before all subsystems have been
          * registered, no tasks have been forked, so we don't
@@ -2789,37 +2787,6 @@ void cgroup_fork_callbacks(struct task_struct *child)
         }
 }
 
-#ifdef CONFIG_MM_OWNER
-/**
- * cgroup_mm_owner_callbacks - run callbacks when the mm->owner changes
- * @p: the new owner
- *
- * Called on every change to mm->owner. mm_init_owner() does not
- * invoke this routine, since it assigns the mm->owner the first time
- * and does not change it.
- *
- * The callbacks are invoked with mmap_sem held in read mode.
- */
-void cgroup_mm_owner_callbacks(struct task_struct *old, struct task_struct *new)
-{
-        struct cgroup *oldcgrp, *newcgrp = NULL;
-
-        if (need_mm_owner_callback) {
-                int i;
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
-                        struct cgroup_subsys *ss = subsys[i];
-                        oldcgrp = task_cgroup(old, ss->subsys_id);
-                        if (new)
-                                newcgrp = task_cgroup(new, ss->subsys_id);
-                        if (oldcgrp == newcgrp)
-                                continue;
-                        if (ss->mm_owner_changed)
-                                ss->mm_owner_changed(ss, oldcgrp, newcgrp, new);
-                }
-        }
-}
-#endif /* CONFIG_MM_OWNER */
-
 /**
  * cgroup_post_fork - called on a new task after adding it to the task list
  * @child: the task in question
diff --git a/kernel/exit.c b/kernel/exit.c
index c9e5a1c14e08..f923724ab3c9 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -642,35 +642,31 @@ retry:
         /*
          * We found no owner yet mm_users > 1: this implies that we are
          * most likely racing with swapoff (try_to_unuse()) or /proc or
-         * ptrace or page migration (get_task_mm()).  Mark owner as NULL,
-         * so that subsystems can understand the callback and take action.
+         * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
          */
-        down_write(&mm->mmap_sem);
-        cgroup_mm_owner_callbacks(mm->owner, NULL);
         mm->owner = NULL;
-        up_write(&mm->mmap_sem);
         return;
 
 assign_new_owner:
         BUG_ON(c == p);
         get_task_struct(c);
-        read_unlock(&tasklist_lock);
-        down_write(&mm->mmap_sem);
         /*
          * The task_lock protects c->mm from changing.
          * We always want mm->owner->mm == mm
          */
         task_lock(c);
+        /*
+         * Delay read_unlock() till we have the task_lock()
+         * to ensure that c does not slip away underneath us
+         */
+        read_unlock(&tasklist_lock);
         if (c->mm != mm) {
                 task_unlock(c);
-                up_write(&mm->mmap_sem);
                 put_task_struct(c);
                 goto retry;
         }
-        cgroup_mm_owner_callbacks(mm->owner, c);
         mm->owner = c;
         task_unlock(c);
-        up_write(&mm->mmap_sem);
         put_task_struct(c);
 }
 #endif /* CONFIG_MM_OWNER */