[PATCH 1/1] exit: change the release_task() paths to call flush_sigqueue() lockless

[Oleg Nesterov <oleg@redhat.com>]

A task can block a signal, accumulate up to RLIMIT_SIGPENDING sigqueues,
and exit. In this case __exit_signal()->flush_sigqueue() called with irqs
disabled can triger a hard lockup, see
https://lore.kernel.org/all/20190322114917.GC28876@redhat.com/

Fortunately, after the recent posixtimer changes sys_timer_delete() paths
no longer try to clear SIGQUEUE_PREALLOC and/or free tmr->sigq, and after
the exiting task passes __exit_signal() lock_task_sighand() can't succeed
and pid_task(tmr->it_pid) will return NULL.

This means that after __exit_signal(tsk) nobody can play with tsk->pending
or (if group_dead) with tsk->signal->shared_pending, so release_task() can
safely call flush_sigqueue() after write_unlock_irq(&tasklist_lock).

Also, kill clear_tsk_thread_flag(TIF_SIGPENDING), it was never needed.

TODO:
	- we can probably shift posix_cpu_timers_exit() as well
	- do_sigaction() can hit the similar problem

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 3485e5fc499e..bc2c24ea4181 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -200,20 +200,12 @@ static void __exit_signal(struct task_struct *tsk)
 	__unhash_process(tsk, group_dead);
 	write_sequnlock(&sig->stats_lock);
 
-	/*
-	 * Do this under ->siglock, we can race with another thread
-	 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
-	 */
-	flush_sigqueue(&tsk->pending);
 	tsk->sighand = NULL;
 	spin_unlock(&sighand->siglock);
 
 	__cleanup_sighand(sighand);
-	clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
-	if (group_dead) {
-		flush_sigqueue(&sig->shared_pending);
+	if (group_dead)
 		tty_kref_put(tty);
-	}
 }
 
 static void delayed_put_task_struct(struct rcu_head *rhp)
@@ -279,6 +271,11 @@ void release_task(struct task_struct *p)
 	proc_flush_pid(thread_pid);
 	put_pid(thread_pid);
 	release_thread(p);
+
+	flush_sigqueue(&p->pending);
+	if (thread_group_leader(p))
+		flush_sigqueue(&p->signal->shared_pending);
+
 	put_task_struct_rcu_user(p);
 
 	p = leader;
-- 
2.25.1.362.g51ebf55