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cgroup: simplify threadgroup locking

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Note: This commit was originally committed as b5ba75b5fc but got
      reverted by f9f9e7b776 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 <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
This commit is contained in:
Tejun Heo 2015-09-16 13:03:02 -04:00
parent 1ed1328792
commit 3014dde762
1 changed files with 12 additions and 33 deletions
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45
kernel/cgroup.c
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@ -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;
}