From 3014dde762f618fbcfe899f9ce9e929a2e5aa6dd Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 16 Sep 2015 13:03:02 -0400 Subject: [PATCH] cgroup: simplify threadgroup locking Note: This commit was originally committed as b5ba75b5fc0e but got reverted by f9f9e7b77614 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: Tejun Heo Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com --- kernel/cgroup.c | 45 ++++++++++++--------------------------------- 1 file changed, 12 insertions(+), 33 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 115091efa889..2cf0f79f1fc9 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -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; } -- 2.20.1