Commit 3014dde7 authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: simplify threadgroup locking 



Note: This commit was originally committed as b5ba75b5 but got
      reverted by f9f9e7b7 due to the performance regression from
      the percpu_rwsem write down/up operations added to cgroup task
      migration path.  percpu_rwsem changes which alleviate the
      performance issue are pending for v4.4-rc1 merge window.
      Re-apply.

Now that threadgroup locking is made global, code paths around it can
be simplified.

* lock-verify-unlock-retry dancing removed from __cgroup_procs_write().

* Race protection against de_thread() removed from
  cgroup_update_dfl_csses().

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
parent 1ed13287

kernel/cgroup.c

+12 −33
Original line number Diff line number Diff line
@@ -2460,14 +2460,13 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf, 
	if (!cgrp)

		return -ENODEV;



retry_find_task:

	percpu_down_write(&cgroup_threadgroup_rwsem);

	rcu_read_lock();

	if (pid) {

		tsk = find_task_by_vpid(pid);

		if (!tsk) {

			rcu_read_unlock();

			ret = -ESRCH;

			goto out_unlock_cgroup;

			goto out_unlock_rcu;

		}

	} else {

		tsk = current;
@@ -2483,37 +2482,23 @@ retry_find_task: 
	 */

	if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {

		ret = -EINVAL;

		rcu_read_unlock();

		goto out_unlock_cgroup;

		goto out_unlock_rcu;

	}



	get_task_struct(tsk);

	rcu_read_unlock();



	percpu_down_write(&cgroup_threadgroup_rwsem);

	if (threadgroup) {

		if (!thread_group_leader(tsk)) {

			/*

			 * a race with de_thread from another thread's exec()

			 * may strip us of our leadership, if this happens,

			 * there is no choice but to throw this task away and

			 * try again; this is

			 * "double-double-toil-and-trouble-check locking".

			 */

			percpu_up_write(&cgroup_threadgroup_rwsem);

			put_task_struct(tsk);

			goto retry_find_task;

		}

	}



	ret = cgroup_procs_write_permission(tsk, cgrp, of);

	if (!ret)

		ret = cgroup_attach_task(cgrp, tsk, threadgroup);



	percpu_up_write(&cgroup_threadgroup_rwsem);



	put_task_struct(tsk);

out_unlock_cgroup:

	goto out_unlock_threadgroup;



out_unlock_rcu:

	rcu_read_unlock();

out_unlock_threadgroup:

	percpu_up_write(&cgroup_threadgroup_rwsem);

	cgroup_kn_unlock(of->kn);

	return ret ?: nbytes;

}
@@ -2658,6 +2643,8 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp) 


	lockdep_assert_held(&cgroup_mutex);



	percpu_down_write(&cgroup_threadgroup_rwsem);



	/* look up all csses currently attached to @cgrp's subtree */

	down_read(&css_set_rwsem);

	css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {
@@ -2713,17 +2700,8 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp) 
				goto out_finish;

			last_task = task;



			percpu_down_write(&cgroup_threadgroup_rwsem);

			/* raced against de_thread() from another thread? */

			if (!thread_group_leader(task)) {

				percpu_up_write(&cgroup_threadgroup_rwsem);

				put_task_struct(task);

				continue;

			}



			ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);



			percpu_up_write(&cgroup_threadgroup_rwsem);

			put_task_struct(task);



			if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
@@ -2733,6 +2711,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp) 


out_finish:

	cgroup_migrate_finish(&preloaded_csets);

	percpu_up_write(&cgroup_threadgroup_rwsem);

	return ret;

}

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