[PATCH 1/1] exit: kill signal_struct->quick_threads

[Oleg Nesterov <oleg@redhat.com>]

do_exit() can simply decrement signal_struct->live earlier and call
synchronize_group_exit() only if group_dead is true. Also, this code
can avoid spin_lock_irq(siglock) if SIGNAL_GROUP_EXIT is already set.

The only "nontrivial" user of signal->live is css_task_iter_advance()
but it should not be affected. With or without this change the task
is the exiting/exited leader which has already passed cgroup_exit(),
atomic_read(signal->live) can race with sub-threads but this is fine.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/sched/signal.h |  1 -
 kernel/exit.c                | 12 +++++++-----
 kernel/fork.c                |  2 --
 3 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 0a0e23c45406..f98aae40d7e6 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -95,7 +95,6 @@ struct signal_struct {
 	refcount_t		sigcnt;
 	atomic_t		live;
 	int			nr_threads;
-	int			quick_threads;
 	struct list_head	thread_head;
 
 	wait_queue_head_t	wait_chldexit;	/* for wait4() */
diff --git a/kernel/exit.c b/kernel/exit.c
index f95a2c1338a8..11bac37e2bc6 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -800,10 +800,11 @@ static void synchronize_group_exit(struct task_struct *tsk, long code)
 	struct sighand_struct *sighand = tsk->sighand;
 	struct signal_struct *signal = tsk->signal;
 
+	if (READ_ONCE(signal->flags) & SIGNAL_GROUP_EXIT)
+		return;
+
 	spin_lock_irq(&sighand->siglock);
-	signal->quick_threads--;
-	if ((signal->quick_threads == 0) &&
-	    !(signal->flags & SIGNAL_GROUP_EXIT)) {
+	if (!(signal->flags & SIGNAL_GROUP_EXIT)) {
 		signal->flags = SIGNAL_GROUP_EXIT;
 		signal->group_exit_code = code;
 		signal->group_stop_count = 0;
@@ -818,7 +819,9 @@ void __noreturn do_exit(long code)
 
 	WARN_ON(irqs_disabled());
 
-	synchronize_group_exit(tsk, code);
+	group_dead = atomic_dec_and_test(&tsk->signal->live);
+	if (group_dead)
+		synchronize_group_exit(tsk, code);
 
 	WARN_ON(tsk->plug);
 
@@ -833,7 +836,6 @@ void __noreturn do_exit(long code)
 	exit_signals(tsk);  /* sets PF_EXITING */
 
 	acct_update_integrals(tsk);
-	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
 		/*
 		 * If the last thread of global init has exited, panic
diff --git a/kernel/fork.c b/kernel/fork.c
index 99076dbe27d8..4c361d2bdc12 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1861,7 +1861,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 		return -ENOMEM;
 
 	sig->nr_threads = 1;
-	sig->quick_threads = 1;
 	atomic_set(&sig->live, 1);
 	refcount_set(&sig->sigcnt, 1);
 
@@ -2590,7 +2589,6 @@ __latent_entropy struct task_struct *copy_process(
 			__this_cpu_inc(process_counts);
 		} else {
 			current->signal->nr_threads++;
-			current->signal->quick_threads++;
 			atomic_inc(&current->signal->live);
 			refcount_inc(&current->signal->sigcnt);
 			task_join_group_stop(p);
-- 
2.25.1.362.g51ebf55