cgroup: pids: kill pids_fork(), simplify pids_can_fork() and pids_cancel_fork()

Now that we know that the forking task can't migrate amd the child is always
moved to the same cgroup by cgroup_post_fork()->css_set_move_task() we can
change pids_can_fork() and pids_cancel_fork() to just use task_css(current).
And since we no longer need to pin this css, we can remove pid_fork().

Note: the patch uses task_css_check(true), perhaps it makes sense to add a
helper or change task_css_set_check() to take cgroup_threadgroup_rwsem into
account.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
index 15ef2e46c396bb4e429f7a1acaca64b17b115da9..de3359a48dbb2b4d02372c59fd3e9f51897fddce 100644 (file)
--- a/kernel/cgroup_pids.c
+++ b/kernel/cgroup_pids.c
@@ -205,48 +205,28 @@ static void pids_cancel_attach(struct cgroup_subsys_state *css,
        }
 }
 
+/*
+ * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
+ * on threadgroup_change_begin() held by the copy_process().
+ */
 static int pids_can_fork(struct task_struct *task, void **priv_p)
 {
        struct cgroup_subsys_state *css;
        struct pids_cgroup *pids;
-       int err;
 
-       /*
-        * Use the "current" task_css for the pids subsystem as the tentative
-        * css. It is possible we will charge the wrong hierarchy, in which
-        * case we will forcefully revert/reapply the charge on the right
-        * hierarchy after it is committed to the task proper.
-        */
-       css = task_get_css(current, pids_cgrp_id);
+       css = task_css_check(current, pids_cgrp_id, true);
        pids = css_pids(css);
-
-       err = pids_try_charge(pids, 1);
-       if (err)
-               goto err_css_put;
-
-       *priv_p = css;
-       return 0;
-
-err_css_put:
-       css_put(css);
-       return err;
+       return pids_try_charge(pids, 1);
 }
 
 static void pids_cancel_fork(struct task_struct *task, void *priv)
 {
-       struct cgroup_subsys_state *css = priv;
-       struct pids_cgroup *pids = css_pids(css);
+       struct cgroup_subsys_state *css;
+       struct pids_cgroup *pids;
 
+       css = task_css_check(current, pids_cgrp_id, true);
+       pids = css_pids(css);
        pids_uncharge(pids, 1);
-       css_put(css);
-}
-
-static void pids_fork(struct task_struct *task, void *priv)
-{
-       struct cgroup_subsys_state *css = priv;
-
-       WARN_ON(task_css_check(task, pids_cgrp_id, true) != css);
-       css_put(css);
 }
 
 static void pids_free(struct task_struct *task)
@@ -329,7 +309,6 @@ struct cgroup_subsys pids_cgrp_subsys = {
        .cancel_attach  = pids_cancel_attach,
        .can_fork       = pids_can_fork,
        .cancel_fork    = pids_cancel_fork,
-       .fork           = pids_fork,
        .free           = pids_free,
        .legacy_cftypes = pids_files,
        .dfl_cftypes    = pids_files,