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, 2 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 9968332cceed..81f58b4a5418 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -121,6 +121,7 @@ extern int cgroup_can_fork(struct task_struct *p);
 extern void cgroup_cancel_fork(struct task_struct *p);
 extern void cgroup_post_fork(struct task_struct *p);
 void cgroup_exit(struct task_struct *p);
+void cgroup_release(struct task_struct *p);
 void cgroup_free(struct task_struct *p);
 
 int cgroup_init_early(void);
@@ -697,6 +698,7 @@ static inline int cgroup_can_fork(struct task_struct *p) { return 0; }
 static inline void cgroup_cancel_fork(struct task_struct *p) {}
 static inline void cgroup_post_fork(struct task_struct *p) {}
 static inline void cgroup_exit(struct task_struct *p) {}
+static inline void cgroup_release(struct task_struct *p) {}
 static inline void cgroup_free(struct task_struct *p) {}
 
 static inline int cgroup_init_early(void) { return 0; }