linux-kernel - [PATCH 2/3] cgroup: Iterate tasks that did not finish do

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Date:   Fri, 17 Jan 2020 16:15:32 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     cgroups@...r.kernel.org
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
        alex.shi@...ux.alibaba.com, guro@...com, kernel-team@...roid.com,
        linger.lee@...iatek.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, matthias.bgg@...il.com,
        shuah@...nel.org, tomcherry@...gle.com
Subject: [PATCH 2/3] cgroup: Iterate tasks that did not finish do_exit()

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>
---
 kernel/cgroup/cgroup.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index b56283e13491..132d258e7172 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4492,11 +4492,12 @@ 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_list == CSS_SET_TASKS_DYING &&
+		    !atomic_read(&task->signal->live))
 			goto repeat;
 	} else {
 		/* skip all dying ones */
-		if (task->flags & PF_EXITING)
+		if (it->cur_list == CSS_SET_TASKS_DYING)
 			goto repeat;
 	}
 }
-- 
2.24.1