cgroup/pids: turn cgroup_subsys->free() into cgroup_subsys->release() to fix the accounting

The only user of cgroup_subsys->free() callback is pids_cgrp_subsys which
needs pids_free() to uncharge the pid.

However, ->free() is called from __put_task_struct()->cgroup_free() and this
is too late. Even the trivial program which does

	for (;;) {
		int pid = fork();
		assert(pid >= 0);
		if (pid)
			wait(NULL);
		else
			exit(0);
	}

can run out of limits because release_task()->call_rcu(delayed_put_task_struct)
implies an RCU gp after the task/pid goes away and before the final put().

Test-case:

	mkdir -p /tmp/CG
	mount -t cgroup2 none /tmp/CG
	echo '+pids' > /tmp/CG/cgroup.subtree_control

	mkdir /tmp/CG/PID
	echo 2 > /tmp/CG/PID/pids.max

	perl -e 'while ($p = fork) { wait; } $p // die "fork failed: $!\n"' &
	echo $! > /tmp/CG/PID/cgroup.procs

Without this patch the forking process fails soon after migration.

Rename cgroup_subsys->free() to cgroup_subsys->release() and move the callsite
into the new helper, cgroup_release(), called by release_task() which actually
frees the pid(s).

Reported-by: Herton R. Krzesinski <hkrzesin@redhat.com>
Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 8fcbae1b8db0..120d1d40704b 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -602,7 +602,7 @@ struct cgroup_subsys {
 	void (*cancel_fork)(struct task_struct *task);
 	void (*fork)(struct task_struct *task);
 	void (*exit)(struct task_struct *task);
-	void (*free)(struct task_struct *task);
+	void (*release)(struct task_struct *task);
 	void (*bind)(struct cgroup_subsys_state *root_css);
 
 	bool early_init:1;