change thread_group_empty() to use task_struct->thread_node

Patch series "kill task_struct->thread_group".


This patch (of 2):

It could use list_is_singular() but this way it is cheaper.  Plus the
thread_group_leader() check makes it clear that thread_group_empty() can
only return true if p is a group leader.  This was not immediately obvious
before this patch.

task_struct->thread_group no longer has users, it can die.

Link: https://lkml.kernel.org/r/20230826111200.GA22982@redhat.com
Link: https://lkml.kernel.org/r/20230826111406.GA23238@redhat.comSigned-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/sched/signal.h

@@ -733,7 +733,8 @@ static inline struct task_struct *next_thread(struct task_struct *p)
 
 static inline int thread_group_empty(struct task_struct *p)
 {
-	return list_empty(&p->thread_group);
+	return thread_group_leader(p) &&
+	       list_is_last(&p->thread_node, &p->signal->thread_head);
 }
 
 #define delay_group_leader(p) \