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SubjectRe: [PATCH 2/3] cgroup: remove double-checking locking from attach_task_by_pid
This is not going to work. Will fix up and resend. Need to goto
the top as suggested by Tejun.

Mandeep Singh Baines (msb@chromium.org) wrote:
> By reading group_leader after taking the threadgroup_lock, we can
> avoid the double-check locking. This removes the return of
> -EAGAIN so we can cleanup cgroup_procs_write at the same time.
>
> The suggestion was made here:
>
> https://lkml.org/lkml/2011/12/22/371
>
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: containers@lists.linux-foundation.org
> Cc: cgroups@vger.kernel.org
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> ---
> kernel/cgroup.c | 40 ++++++----------------------------------
> 1 files changed, 6 insertions(+), 34 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 032139d..a5f7d1b 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2234,9 +2234,6 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
> ret= -ESRCH;
> goto out_unlock_cgroup;
> }
> - /* we check later for a group_leader race with de_thread */
> - if (threadgroup)
> - tsk = tsk->group_leader;
> /*
> * even if we're attaching all tasks in the thread group, we
> * only need to check permissions on one of them.
> @@ -2252,33 +2249,17 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
> get_task_struct(tsk);
> rcu_read_unlock();
> } else {
> - if (threadgroup)
> - tsk = current->group_leader;
> - else
> - tsk = current;
> + tsk = current;
> get_task_struct(tsk);
> }
>
> threadgroup_lock(tsk);
> -
> - 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 (from cgroup_procs_write); this is
> - * "double-double-toil-and-trouble-check locking".
> - */
> - ret = -EAGAIN;
> - goto out_unlock_threadgroup;
> - }
> - ret = cgroup_attach_proc(cgrp, tsk);
> - } else
> + if (threadgroup)
> + ret = cgroup_attach_proc(cgrp, tsk->group_leader);
> + else
> ret = cgroup_attach_task(cgrp, tsk);
> -
> -out_unlock_threadgroup:
> threadgroup_unlock(tsk);
> +
> put_task_struct(tsk);
> out_unlock_cgroup:
> cgroup_unlock();
> @@ -2292,16 +2273,7 @@ static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
>
> static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
> {
> - int ret;
> - do {
> - /*
> - * attach_proc fails with -EAGAIN if threadgroup leadership
> - * changes in the middle of the operation, in which case we need
> - * to find the task_struct for the new leader and start over.
> - */
> - ret = attach_task_by_pid(cgrp, tgid, true);
> - } while (ret == -EAGAIN);
> - return ret;
> + return attach_task_by_pid(cgrp, tgid, true);
> }
>
> /**
> --
> 1.7.3.1
>


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