linux-kernel - [tip:sched/urgent] sched/autogroup: Do not use autogroup->tg in zombie threads

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Date:   Tue, 22 Nov 2016 04:29:21 -0800
From:   tip-bot for Oleg Nesterov <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     efault@....de, tglx@...utronix.de, oleg@...hat.com,
        mingo@...nel.org, hpa@...or.com, peterz@...radead.org,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: [tip:sched/urgent] sched/autogroup: Do not use autogroup->tg in
 zombie threads

Commit-ID:  8e5bfa8c1f8471aa4a2d30be631ef2b50e10abaf
Gitweb:     http://git.kernel.org/tip/8e5bfa8c1f8471aa4a2d30be631ef2b50e10abaf
Author:     Oleg Nesterov <oleg@...hat.com>
AuthorDate: Mon, 14 Nov 2016 19:46:12 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 22 Nov 2016 12:33:43 +0100

sched/autogroup: Do not use autogroup->tg in zombie threads

Exactly because for_each_thread() in autogroup_move_group() can't see it
and update its ->sched_task_group before _put() and possibly free().

So the exiting task needs another sched_move_task() before exit_notify()
and we need to re-introduce the PF_EXITING (or similar) check removed by
the previous change for another reason.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: hartsjc@...hat.com
Cc: vbendel@...hat.com
Cc: vlovejoy@...hat.com
Link: http://lkml.kernel.org/r/20161114184612.GA15968@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/linux/sched.h     |  2 ++
 kernel/exit.c             |  1 +
 kernel/sched/auto_group.c | 19 +++++++++++++++++++
 3 files changed, 22 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 348f51b..e9c009d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2567,6 +2567,7 @@ extern void sched_autogroup_create_attach(struct task_struct *p);
 extern void sched_autogroup_detach(struct task_struct *p);
 extern void sched_autogroup_fork(struct signal_struct *sig);
 extern void sched_autogroup_exit(struct signal_struct *sig);
+extern void sched_autogroup_exit_task(struct task_struct *p);
 #ifdef CONFIG_PROC_FS
 extern void proc_sched_autogroup_show_task(struct task_struct *p, struct seq_file *m);
 extern int proc_sched_autogroup_set_nice(struct task_struct *p, int nice);
@@ -2576,6 +2577,7 @@ static inline void sched_autogroup_create_attach(struct task_struct *p) { }
 static inline void sched_autogroup_detach(struct task_struct *p) { }
 static inline void sched_autogroup_fork(struct signal_struct *sig) { }
 static inline void sched_autogroup_exit(struct signal_struct *sig) { }
+static inline void sched_autogroup_exit_task(struct task_struct *p) { }
 #endif
 
 extern int yield_to(struct task_struct *p, bool preempt);
diff --git a/kernel/exit.c b/kernel/exit.c
index 9d68c45..3076f30 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -836,6 +836,7 @@ void __noreturn do_exit(long code)
 	 */
 	perf_event_exit_task(tsk);
 
+	sched_autogroup_exit_task(tsk);
 	cgroup_exit(tsk);
 
 	/*
diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index ad2b19a..f1c8fd5 100644
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c
@@ -115,10 +115,26 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
 	 * If we race with autogroup_move_group() the caller can use the old
 	 * value of signal->autogroup but in this case sched_move_task() will
 	 * be called again before autogroup_kref_put().
+	 *
+	 * However, there is no way sched_autogroup_exit_task() could tell us
+	 * to avoid autogroup->tg, so we abuse PF_EXITING flag for this case.
 	 */
+	if (p->flags & PF_EXITING)
+		return false;
+
 	return true;
 }
 
+void sched_autogroup_exit_task(struct task_struct *p)
+{
+	/*
+	 * We are going to call exit_notify() and autogroup_move_group() can't
+	 * see this thread after that: we can no longer use signal->autogroup.
+	 * See the PF_EXITING check in task_wants_autogroup().
+	 */
+	sched_move_task(p);
+}
+
 static void
 autogroup_move_group(struct task_struct *p, struct autogroup *ag)
 {
@@ -142,6 +158,9 @@ autogroup_move_group(struct task_struct *p, struct autogroup *ag)
 	 * In the latter case for_each_thread() can not miss a migrating thread,
 	 * cpu_cgroup_attach() must not be possible after cgroup_exit() and it
 	 * can't be removed from thread list, we hold ->siglock.
+	 *
+	 * If an exiting thread was already removed from thread list we rely on
+	 * sched_autogroup_exit_task().
 	 */
 	for_each_thread(p, t)
 		sched_move_task(t);