* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2010-12-24 16:59:13]: > On Fri, 2010-12-24 at 13:16 +0100, Mike Galbraith wrote: > > On Fri, 2010-12-24 at 11:54 +0100, Peter Zijlstra wrote: > > > > Right, so the cgroup core is supposed to already emit -EBUSY when there > > > are associated tasks with the cgroup, that _should_ be sufficient, the > > > pre_destroy() method is to frob some extra constraints or somesuch. > > > > > > Our problem looks to be that a task (afaict usually current) changes > > > cgroups without us getting notified of it. On destruction the task is > > > still enqueued in the cfs_rq being destroyed but is not actually part of > > > that cgroup according to the task->css bits. > > > > Could it be an exiting task? We're still preemptible, and iirc, you run > > a CONFIG_PREEMPT kernel. (grasp at all straws;) > > > > cgroup_exit: > > /* Reassign the task to the init_css_set. */ > > task_lock(tsk); > > cg = tsk->cgroups; > > tsk->cgroups = &init_css_set; > > task_unlock(tsk); > > if (cg) > > put_css_set_taskexit(cg); > > > > This straw appears true: > > $ grep -e cpu_cgroup\\\|f491447c log9 > > ... > > kworker/-1196 0d..2. 1601180us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service > kworker/-1196 0d..2. 1601186us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service > kworker/-1196 0d..2. 1601188us : __dequeue_entity: f491447c from f492a480, 1 left > kworker/-1196 0d..2. 1601188us : pick_next_task_fair: picked: f491447c, modprobe/1210 > kworker/-1196 0d..2. 1601192us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service > modprobe-1210 0d..5. 1601802us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..5. 1601807us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601817us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601819us : __enqueue_entity: f491447c to f492a480, 1 tasks > modprobe-1210 0d..2. 1601826us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601832us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601839us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601848us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601854us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601860us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601865us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601871us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > kworker/-1196 0d..2. 1601872us : __dequeue_entity: f491447c from f492a480, 1 left > kworker/-1196 0d..2. 1601873us : pick_next_task_fair: picked: f491447c, modprobe/1210 > kworker/-1196 0d..2. 1601876us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: / > modprobe-1210 0d..7. 1601895us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..7. 1601900us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601909us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601911us : __enqueue_entity: f491447c to f492a480, 1 tasks > modprobe-1210 0d..2. 1601918us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601924us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..2. 1601931us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602071us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602080us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602089us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602097us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602105us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > kworker/-1196 0d..2. 1602107us : __dequeue_entity: f491447c from f492a480, 1 left > kworker/-1196 0d..2. 1602108us : pick_next_task_fair: picked: f491447c, modprobe/1210 > kworker/-1196 0d..2. 1602114us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: / > modprobe-1210 0d..3. 1602128us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 80, load: 1024, cgroup: / > > > So cgroup moves a task without calling cgroup_subsys::attach() which is > odd, but it does have an ::exit method, sadly it calls that _before_ > re-assigning the task, which means we have to jump through some hoops. > > The below seems to fix the problem for me.. > > --- > Subject: sched, cgroup: Use exit hook to avoid use-after-free crash > > By not notifying the controller of the on-exit move back to > init_css_set, we fail to move the task out of the previous cgroup's > cfs_rq. This leads to an opportunity for a cgroup-destroy to come in and > free the cgroup (there are no active tasks left in it after all) to > which the not-quite dead task is still enqueued. > > Cc: stable@xxxxxxxxxx > Reported-by: Miklos Vajna <vmiklos@xxxxxxxxxxxxxx> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > --- > kernel/sched.c | 10 ++++++++++ > 1 files changed, 10 insertions(+), 0 deletions(-) > > diff --git a/kernel/sched.c b/kernel/sched.c > index 7e401f8..572625c 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -611,6 +611,9 @@ static inline struct task_group *task_group(struct task_struct *p) > struct task_group *tg; > struct cgroup_subsys_state *css; > > + if (p->flags & PF_EXITING) > + return &root_task_group; > + > css = task_subsys_state_check(p, cpu_cgroup_subsys_id, > lockdep_is_held(&task_rq(p)->lock)); > tg = container_of(css, struct task_group, css); > @@ -8887,6 +8890,12 @@ cpu_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp, > } > } > > +static void > +cpu_cgroup_exit(struct cgroup_subsys *ss, struct task_struct *task) > +{ > + sched_move_task(task); > +} > + > #ifdef CONFIG_FAIR_GROUP_SCHED > static int cpu_shares_write_u64(struct cgroup *cgrp, struct cftype *cftype, > u64 shareval) > @@ -8959,6 +8968,7 @@ struct cgroup_subsys cpu_cgroup_subsys = { > .destroy = cpu_cgroup_destroy, > .can_attach = cpu_cgroup_can_attach, > .attach = cpu_cgroup_attach, > + .exit = cpu_cgroup_exit, > .populate = cpu_cgroup_populate, > .subsys_id = cpu_cgroup_subsys_id, > .early_init = 1, > > Very good catch! Looks very reasonable and correct to me Acked-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> -- Three Cheers, Balbir -- To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html