cgroup: drop task_lock() protection around task->cgroups
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>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fa0567f..f783af9 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -80,12 +80,21 @@
/*
* 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 @@
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 @@
}
/*
- * 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 @@
* 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 @@
*/
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 @@
get_css_set(cset);
}
spin_unlock_irq(&p->sighand->siglock);
-
- task_unlock(p);
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
out_unlock:
@@ -1743,11 +1726,7 @@
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);
/*
@@ -1999,8 +1978,7 @@
* @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)
@@ -2034,7 +2012,7 @@
/*
* 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)
{
@@ -4155,12 +4133,6 @@
* 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 */
@@ -4310,15 +4282,12 @@
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);
}
@@ -4347,27 +4316,13 @@
* 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)
{
@@ -4377,20 +4332,17 @@
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);
@@ -4405,7 +4357,6 @@
}
}
}
- task_unlock(tsk);
if (put_cset)
put_css_set(cset, true);
