linux-kernel - Re: [PATCH v2 1/3] cgroup: Iterate tasks that did not finish do

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Date:   Wed, 5 Feb 2020 09:27:45 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     cgroups mailinglist <cgroups@...r.kernel.org>,
        alex.shi@...ux.alibaba.com, Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        kernel-team <kernel-team@...roid.com>,
        JeiFeng Lee <linger.lee@...iatek.com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, Li Zefan <lizefan@...wei.com>,
        matthias.bgg@...il.com, shuah@...nel.org,
        Tejun Heo <tj@...nel.org>, Tom Cherry <tomcherry@...gle.com>
Subject: Re: [PATCH v2 1/3] cgroup: Iterate tasks that did not finish do_exit()

On Fri, Jan 24, 2020 at 2:56 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Fri, Jan 24, 2020 at 3:40 AM Michal Koutný <mkoutny@...e.com> wrote:
> >
> > PF_EXITING is set earlier than actual removal from css_set when a task
> > is exitting. This can confuse cgroup.procs readers who see no PF_EXITING
> > tasks, however, rmdir is checking against css_set membership so it can
> > transitionally fail with EBUSY.
> >
> > Fix this by listing tasks that weren't unlinked from css_set active
> > lists.
> > It may happen that other users of the task iterator (without
> > CSS_TASK_ITER_PROCS) spot a PF_EXITING task before cgroup_exit(). This
> > is equal to the state before commit c03cd7738a83 ("cgroup: Include dying
> > leaders with live threads in PROCS iterations") but it may be reviewed
> > later.
> >
> > Reported-by: Suren Baghdasaryan <surenb@...gle.com>
> > Fixes: c03cd7738a83 ("cgroup: Include dying leaders with live threads in PROCS iterations")
> > Signed-off-by: Michal Koutný <mkoutny@...e.com>
> > ---
> >  include/linux/cgroup.h |  1 +
> >  kernel/cgroup/cgroup.c | 23 ++++++++++++++++-------
> >  2 files changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> > index d7ddebd0cdec..e75d2191226b 100644
> > --- a/include/linux/cgroup.h
> > +++ b/include/linux/cgroup.h
> > @@ -62,6 +62,7 @@ struct css_task_iter {
> >         struct list_head                *mg_tasks_head;
> >         struct list_head                *dying_tasks_head;
> >
> > +       struct list_head                *cur_tasks_head;
> >         struct css_set                  *cur_cset;
> >         struct css_set                  *cur_dcset;
> >         struct task_struct              *cur_task;
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 735af8f15f95..a6e3619e013b 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -4404,12 +4404,16 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
> >                 }
> >         } while (!css_set_populated(cset) && list_empty(&cset->dying_tasks));
> >
> > -       if (!list_empty(&cset->tasks))
> > +       if (!list_empty(&cset->tasks)) {
> >                 it->task_pos = cset->tasks.next;
> > -       else if (!list_empty(&cset->mg_tasks))
> > +               it->cur_tasks_head = &cset->tasks;
> > +       } else if (!list_empty(&cset->mg_tasks)) {
> >                 it->task_pos = cset->mg_tasks.next;
> > -       else
> > +               it->cur_tasks_head = &cset->mg_tasks;
> > +       } else {
> >                 it->task_pos = cset->dying_tasks.next;
> > +               it->cur_tasks_head = &cset->dying_tasks;
> > +       }
> >
> >         it->tasks_head = &cset->tasks;
> >         it->mg_tasks_head = &cset->mg_tasks;
> > @@ -4467,10 +4471,14 @@ static void css_task_iter_advance(struct css_task_iter *it)
> >                 else
> >                         it->task_pos = it->task_pos->next;
> >
> > -               if (it->task_pos == it->tasks_head)
> > +               if (it->task_pos == it->tasks_head) {
> >                         it->task_pos = it->mg_tasks_head->next;
> > -               if (it->task_pos == it->mg_tasks_head)
> > +                       it->cur_tasks_head = it->mg_tasks_head;
> > +               }
> > +               if (it->task_pos == it->mg_tasks_head) {
> >                         it->task_pos = it->dying_tasks_head->next;
> > +                       it->cur_tasks_head = it->dying_tasks_head;
> > +               }
> >                 if (it->task_pos == it->dying_tasks_head)
> >                         css_task_iter_advance_css_set(it);
> >         } else {
> > @@ -4489,11 +4497,12 @@ static void css_task_iter_advance(struct css_task_iter *it)
> >                         goto repeat;
> >
> >                 /* and dying leaders w/o live member threads */
> > -               if (!atomic_read(&task->signal->live))
> > +               if (it->cur_tasks_head == it->dying_tasks_head &&
> > +                   !atomic_read(&task->signal->live))
> >                         goto repeat;
> >         } else {
> >                 /* skip all dying ones */
> > -               if (task->flags & PF_EXITING)
> > +               if (it->cur_tasks_head == it->dying_tasks_head)
> >                         goto repeat;
> >         }
> >  }
> > --
> > 2.24.1
> >
>
> Tested-by: Suren Baghdasaryan <surenb@...gle.com>
>
> Thanks!

Hi Folks,
If this new version looks good could we get an Ack please? I need to
start backporting this fix to Android and would like a confirmation
before doing so.
Thanks!