From 51bee5abeab2058ea5813c5615d6197a23dbf041 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Mon, 28 Jan 2019 17:00:13 +0100 Subject: 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 Reported-by: Jan Stancek Signed-off-by: Oleg Nesterov Signed-off-by: Tejun Heo --- include/linux/cgroup-defs.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/cgroup-defs.h') 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; -- cgit