[PATCH 1/5] sched_show_task: fix unsafe usage of ->real_parent

[Oleg Nesterov]

rcu_read_lock() can not protect p->real_parent if release_task(p) was
already called, change sched_show_task() to check pis_alive() like
other users do.

Note: we need some helpers to cleanup the code like this. And it seems
that that the usage of cpu_curr(cpu) in dump_cpu_task() is not safe too.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/sched/core.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 240157c..1ec75c9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4572,8 +4572,10 @@ void sched_show_task(struct task_struct *p)
 #ifdef CONFIG_DEBUG_STACK_USAGE
 	free = stack_not_used(p);
 #endif
+	ppid = 0;
 	rcu_read_lock();
-	ppid = task_pid_nr(rcu_dereference(p->real_parent));
+	if (pid_alive(p))
+		ppid = task_pid_nr(rcu_dereference(p->real_parent));
 	rcu_read_unlock();
 	printk(KERN_CONT "%5lu %5d %6d 0x%08lx\n", free,
 		task_pid_nr(p), ppid,
-- 
1.5.5.1

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