sched_show_task: fix unsafe usage of ->real_parent

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@redhat.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
Cc: Sterling Alexander <stalexan@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Roland McGrath <roland@hack.frob.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bb398c0c5f08..b5797b78add6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4527,8 +4527,10 @@ 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,