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cgroup: prevent spurious transition into non-frozen state 

If freezing of a cgroup races with waking of a task from
the frozen state (like waiting in vfork() or in do_signal_stop()),
a spurious transition of the cgroup state can happen.

The task enters cgroup_leave_frozen(true), the cgroup->nr_frozen_tasks
counter decrements, and the cgroup is switched to the unfrozen state.

To prevent it, let's reserve cgroup_leave_frozen(true) for
terminating processes and use cgroup_leave_frozen(false) otherwise.

To avoid busy-looping in the signal handling loop waiting
for JOBCTL_TRAP_FREEZE set from the cgroup freezing path,
let's do it explicitly in cgroup_leave_frozen(), if the task
is going to stay frozen.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Roman Gushchin 2019-04-26 10:59:44 -07:00 committed by Tejun Heo
parent 533307dc20
commit cb2c4cd878
2 changed files with 6 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
kernel/cgroup/freezer.c
View File

@ -139,19 +139,13 @@ void cgroup_leave_frozen(bool always_leave)
cgroup_update_frozen(cgrp);
WARN_ON_ONCE(!current->frozen);
current->frozen = false;
} else if (!(current->jobctl & JOBCTL_TRAP_FREEZE)) {
spin_lock(&current->sighand->siglock);
current->jobctl |= JOBCTL_TRAP_FREEZE;
set_thread_flag(TIF_SIGPENDING);
spin_unlock(&current->sighand->siglock);
}
spin_unlock_irq(&css_set_lock);
if (unlikely(current->frozen)) {
/*
* If the task remained in the frozen state,
* make sure it won't reach userspace without
* entering the signal handling loop.
*/
spin_lock_irq(&current->sighand->siglock);
recalc_sigpending();
spin_unlock_irq(&current->sighand->siglock);
}
}
/*

2
kernel/signal.c
View File

@ -2514,7 +2514,7 @@ bool get_signal(struct ksignal *ksig)
*/
if (unlikely(cgroup_task_frozen(current))) {
spin_unlock_irq(&sighand->siglock);
cgroup_leave_frozen(true);
cgroup_leave_frozen(false);
goto relock;
}