Messages in this thread | | | Date | Thu, 16 Mar 2017 17:17:57 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] kthread, cgroup: close race window where new kthreads can be migrated to non-root cgroups |
| |
On 03/16, Tejun Heo wrote: > > Hello, > > On Thu, Mar 16, 2017 at 04:02:34PM +0100, Oleg Nesterov wrote: > > > +bool kthread_initialized(struct task_struct *k) > > > +{ > > > + struct kthread *kthread = to_kthread(k); > > > + > > > + return kthread && test_bit(KTHREAD_INITIALIZED, &kthread->flags); > > > +} > > > > Not sure I understand... > > > > With this patch you can no longer migrate a kernel thread created by > > kernel_thread() ? Note that to_kthread() is NULL unless it was created > > by kthread_create(). > > Yeah, what it does is preventing migration of kthreads until the > kthread owner wakes it up for the first time. The problem is that > kthread_bind() seals up future cgroup migrations from userland but > doesn't move back the kthread to the root cgroup, so the userland has > a window where it can mangle with cgroup membership inbetween and > break things.
This is clear,
> The NULL test is there because the test may be performed before the > kthread itself sets up its struct kthread.
This too.
But this also means that __cgroup_procs_write() will always fail if this task is a kernel thread which was not created by kthread_create().
Currently you can use kernel_thread() (although you shouldn't) and it can be migrated, this won't work after your patch.
Oleg.
| |