[PATCH] cgroup: Remove RCU from task->cgroups

[Colin Cross <ccross@android.com>]

The synchronize_rcu call in cgroup_attach_task can be very
expensive.  All fastpath accesses to task->cgroups already
use task_lock() or cgroup_lock() to protect against updates,
and only the CGROUP_DEBUG files have RCU read-side critical
sections.

This patch replaces rcu_read_lock() with task_lock(current)
around the debug file acceses to current->cgroups and removes
the synchronize_rcu call in cgroup_attach_task.

Signed-off-by: Colin Cross <ccross@android.com>
---
 kernel/cgroup.c |   22 ++++++++--------------
 1 files changed, 8 insertions(+), 14 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 66a416b..4a40183 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -725,14 +725,11 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
  * cgroup_attach_task(), which overwrites one tasks cgroup pointer with
  * another.  It does so using cgroup_mutex, however there are
  * several performance critical places that need to reference
- * task->cgroup without the expense of grabbing a system global
+ * task->cgroups without the expense of grabbing a system global
  * mutex.  Therefore except as noted below, when dereferencing or, as
- * in cgroup_attach_task(), modifying a task'ss cgroup pointer we use
+ * in cgroup_attach_task(), modifying a task's cgroups pointer we use
  * task_lock(), which acts on a spinlock (task->alloc_lock) already in
  * the task_struct routinely used for such matters.
- *
- * P.S.  One more locking exception.  RCU is used to guard the
- * update of a tasks cgroup pointer by cgroup_attach_task()
  */
 
 /**
@@ -1786,7 +1783,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 		retval = -ESRCH;
 		goto out;
 	}
-	rcu_assign_pointer(tsk->cgroups, newcg);
+	tsk->cgroups = newcg;
 	task_unlock(tsk);
 
 	/* Update the css_set linked lists if we're using them */
@@ -1802,7 +1799,6 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 			ss->attach(ss, cgrp, oldcgrp, tsk, false);
 	}
 	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
-	synchronize_rcu();
 	put_css_set(cg);
 
 	/*
@@ -4827,9 +4823,9 @@ static u64 current_css_set_refcount_read(struct cgroup *cont,
 {
 	u64 count;
 
-	rcu_read_lock();
+	task_lock(current);
 	count = atomic_read(&current->cgroups->refcount);
-	rcu_read_unlock();
+	task_unlock(current);
 	return count;
 }
 
@@ -4838,12 +4834,10 @@ static int current_css_set_cg_links_read(struct cgroup *cont,
 					 struct seq_file *seq)
 {
 	struct cg_cgroup_link *link;
-	struct css_set *cg;
 
 	read_lock(&css_set_lock);
-	rcu_read_lock();
-	cg = rcu_dereference(current->cgroups);
-	list_for_each_entry(link, &cg->cg_links, cg_link_list) {
+	task_lock(current);
+	list_for_each_entry(link, &current->cgroups->cg_links, cg_link_list) {
 		struct cgroup *c = link->cgrp;
 		const char *name;
 
@@ -4854,7 +4848,7 @@ static int current_css_set_cg_links_read(struct cgroup *cont,
 		seq_printf(seq, "Root %d group %s\n",
 			   c->root->hierarchy_id, name);
 	}
-	rcu_read_unlock();
+	task_unlock(current);
 	read_unlock(&css_set_lock);
 	return 0;
 }
-- 
1.7.3.1