[PATCH 6/7] cgroup: drop task_lock() protection around task->cgroups

Tejun Heo
2014-02-13T20:29:10Z

For optimization, task_lock() is additionally used to protect
task->cgroups. The optimization is pretty dubious as either
css_set_rwsem is grabbed anyway or PF_EXITING already protects
task->cgroups. It adds only overhead and confusion at this point.
Let's drop task_[un]lock() and update comments accordingly.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/cgroup.h | 6 ++--
kernel/cgroup.c | 97 +++++++++++++-------------------------------------
2 files changed, 28 insertions(+), 75 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 366a2cc..588cc9a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -650,10 +650,12 @@ struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
*/
#ifdef CONFIG_PROVE_RCU
extern struct mutex cgroup_mutex;
+extern struct rw_semaphore css_set_rwsem;
#define task_css_set_check(task, __c) \
rcu_dereference_check((task)->cgroups, \
- lockdep_is_held(&(task)->alloc_lock) || \
- lockdep_is_held(&cgroup_mutex) || (__c))
+ lockdep_is_held(&cgroup_mutex) || \
+ lockdep_is_held(&css_set_rwsem) || \
+ ((task)->flags & PF_EXITING) || (__c))
#else
#define task_css_set_check(task, __c) \
rcu_dereference((task)->cgroups)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1fdc38e..e373520 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -80,12 +80,21 @@ static DEFINE_MUTEX(cgroup_tree_mutex);
/*
* cgroup_mutex is the master lock. Any modification to cgroup or its
* hierarchy must be performed while holding it.
+ *
+ * css_set_rwsem protects task->cgroups pointer, the list of css_set
+ * objects, and the chain of tasks off each css_set.
+ *
+ * These locks are exported if CONFIG_PROVE_RCU so that accessors in
+ * cgroup.h can use them for lockdep annotations.
*/
#ifdef CONFIG_PROVE_RCU
DEFINE_MUTEX(cgroup_mutex);
-EXPORT_SYMBOL_GPL(cgroup_mutex); /* only for lockdep */
+DECLARE_RWSEM(css_set_rwsem);
+EXPORT_SYMBOL_GPL(cgroup_mutex);
+EXPORT_SYMBOL_GPL(css_set_rwsem);
#else
static DEFINE_MUTEX(cgroup_mutex);
+static DECLARE_RWSEM(css_set_rwsem);
#endif

/*
@@ -338,12 +347,6 @@ struct cgrp_cset_link {

static struct css_set init_css_set;
static struct cgrp_cset_link init_cgrp_cset_link;
-
-/*
- * css_set_rwsem protects the list of css_set objects, and the chain of
- * tasks off each css_set.
- */
-static DECLARE_RWSEM(css_set_rwsem);
static int css_set_count;

/*
@@ -803,10 +806,6 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
}

/*
- * There is one global cgroup mutex. We also require taking
- * task_lock() when dereferencing a task's cgroup subsys pointers.
- * See "The task_lock() exception", at the end of this comment.
- *
* A task must hold cgroup_mutex to modify cgroups.
*
* Any task can increment and decrement the count field without lock.
@@ -836,18 +835,6 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
* always has either children cgroups and/or using tasks. So we don't
* need a special hack to ensure that top_cgroup cannot be deleted.
*
- * The task_lock() exception
- *
- * The need for this exception arises from the action of
- * cgroup_attach_task(), which overwrites one task's 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
- * mutex. Therefore except as noted below, when dereferencing or, as
- * in cgroup_attach_task(), modifying a task's cgroup 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()
*/
@@ -1329,8 +1316,6 @@ static void cgroup_enable_task_cg_lists(void)
*/
read_lock(&tasklist_lock);
do_each_thread(g, p) {
- task_lock(p);
-
WARN_ON_ONCE(!list_empty(&p->cg_list) ||
task_css_set(p) != &init_css_set);

@@ -1349,8 +1334,6 @@ static void cgroup_enable_task_cg_lists(void)
get_css_set(cset);
}
spin_unlock_irq(&p->sighand->siglock);
-
- task_unlock(p);
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
out_unlock:
@@ -1741,11 +1724,7 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
old_cset = task_css_set(tsk);

get_css_set(new_cset);
-
- task_lock(tsk);
rcu_assign_pointer(tsk->cgroups, new_cset);
- task_unlock(tsk);
-
list_move(&tsk->cg_list, &new_cset->mg_tasks);

/*
@@ -1997,8 +1976,7 @@ out_release_tset:
* @leader: the task or the leader of the threadgroup to be attached
* @threadgroup: attach the whole threadgroup?
*
- * Call holding cgroup_mutex and the group_rwsem of the leader. Will take
- * task_lock of @tsk or each thread in the threadgroup individually in turn.
+ * Call holding cgroup_mutex and threadgroup_lock of @leader.
*/
static int cgroup_attach_task(struct cgroup *dst_cgrp,
struct task_struct *leader, bool threadgroup)
@@ -2032,7 +2010,7 @@ static int cgroup_attach_task(struct cgroup *dst_cgrp,
/*
* Find the task_struct of the task to attach by vpid and pass it along to the
* function to attach either it or all tasks in its threadgroup. Will lock
- * cgroup_mutex and threadgroup; may take task_lock of task.
+ * cgroup_mutex and threadgroup.
*/
static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
{
@@ -4152,12 +4130,6 @@ core_initcall(cgroup_wq_init);
* proc_cgroup_show()
* - Print task's cgroup paths into seq_file, one line for each hierarchy
* - Used for /proc/<pid>/cgroup.
- * - No need to task_lock(tsk) on this tsk->cgroup reference, as it
- * doesn't really matter if tsk->cgroup changes after we read it,
- * and we take cgroup_mutex, keeping cgroup_attach_task() from changing it
- * anyway. No need to check that tsk->cgroup != NULL, thanks to
- * the_top_cgroup_hack in cgroup_exit(), which sets an exiting tasks
- * cgroup to top_cgroup.
*/

/* TODO: Use a proper seq_file iterator */
@@ -4307,15 +4279,12 @@ void cgroup_post_fork(struct task_struct *child)
struct css_set *cset;

down_write(&css_set_rwsem);
- cset = task_css_set_check(current,
- lockdep_is_held(&css_set_rwsem));
- task_lock(child);
+ cset = task_css_set(current);
if (list_empty(&child->cg_list)) {
rcu_assign_pointer(child->cgroups, cset);
list_add(&child->cg_list, &cset->tasks);
get_css_set(cset);
}
- task_unlock(child);
up_write(&css_set_rwsem);
}

@@ -4344,27 +4313,13 @@ void cgroup_post_fork(struct task_struct *child)
* use notify_on_release cgroups where very high task exit scaling
* is required on large systems.
*
- * the_top_cgroup_hack:
- *
- * Set the exiting tasks cgroup to the root cgroup (top_cgroup).
- *
- * We call cgroup_exit() while the task is still competent to
- * handle notify_on_release(), then leave the task attached to the
- * root cgroup in each hierarchy for the remainder of its exit.
- *
- * To do this properly, we would increment the reference count on
- * top_cgroup, and near the very end of the kernel/exit.c do_exit()
- * code we would add a second cgroup function call, to drop that
- * reference. This would just create an unnecessary hot spot on
- * the top_cgroup reference count, to no avail.
- *
- * Normally, holding a reference to a cgroup without bumping its
- * count is unsafe. The cgroup could go away, or someone could
- * attach us to a different cgroup, decrementing the count on
- * the first cgroup that we never incremented. But in this case,
- * top_cgroup isn't going away, and either task has PF_EXITING set,
- * which wards off any cgroup_attach_task() attempts, or task is a failed
- * fork, never visible to cgroup_attach_task.
+ * We set the exiting tasks cgroup to the root cgroup (top_cgroup). We
+ * call cgroup_exit() while the task is still competent to handle
+ * notify_on_release(), then leave the task attached to the root cgroup in
+ * each hierarchy for the remainder of its exit. No need to bother with
+ * init_css_set refcnting. init_css_set never goes away and we can't race
+ * with migration path - either PF_EXITING is visible to migration path or
+ * @tsk never got on the tasklist.
*/
void cgroup_exit(struct task_struct *tsk, int run_callbacks)
{
@@ -4374,20 +4329,17 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
int i;

/*
- * Unlink from the css_set task list if necessary. Optimistically
- * check cg_list before taking css_set_rwsem.
+ * Unlink from @tsk from its css_set. As migration path can't race
+ * with us, we can check cg_list without grabbing css_set_rwsem.
*/
if (!list_empty(&tsk->cg_list)) {
down_write(&css_set_rwsem);
- if (!list_empty(&tsk->cg_list)) {
- list_del_init(&tsk->cg_list);
- put_cset = true;
- }
+ list_del_init(&tsk->cg_list);
up_write(&css_set_rwsem);
+ put_cset = true;
}

/* Reassign the task to the init_css_set. */
- task_lock(tsk);
cset = task_css_set(tsk);
RCU_INIT_POINTER(tsk->cgroups, &init_css_set);

@@ -4402,7 +4354,6 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
}
}
}
- task_unlock(tsk);

if (put_cset)
put_css_set(cset, true);
--
1.8.5.3