Re: [PATCH] signal: Use list_del_init_careful() in flush_sigqueue()

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Aug 24 2026 at 10:04, Thomas Gleixner wrote:

> On Sat, Aug 22 2026 at 14:37, Hyunwoo Kim wrote:
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index bbc0fd4cc4d7c1..ec9a0a0490d19f 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -482,7 +482,11 @@ void flush_sigqueue(struct sigpending *queue)
>>  	sigemptyset(&queue->signal);
>>  	while (!list_empty(&queue->list)) {
>>  		q = list_entry(queue->list.next, struct sigqueue , list);
>> -		list_del_init(&q->list);
>> +		/*
>> +		 * Pairs with the list_empty() in posixtimer_send_sigqueue().
>
> No. That list_empty() would need to be changed to list_empty_careful()
> to be correct on weakly ordered architectures.
>
> Aside of that I'm not convinced that this is the right way to handle
> this as it cures the symptom and not the underlying problem. Let me
> stare at this some more.

Something like the untested below.

Thanks,

        tglx
---
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -983,6 +983,18 @@ static int de_thread(struct task_struct
 		}
 
 		/*
+		 * Ensure that POSIX timer SIGEV_THREAD_ID signals pending for
+		 * the former leader are removed under sighand::siglock _before_
+		 * taking over the leader's TID. Otherwise the lockless cleanup
+		 * in release_task() can race against a concurrent signal
+		 * delivery to the new leader. The former leader has PF_EXITING
+		 * set which prevents queueing of SIGEV_THREAD_ID signals up to
+		 * the point where it's sighand gets cleared.
+		 */
+		scoped_guard(spinlock_irq, lock)
+			flush_sigqueue(&leader->pending);
+
+		/*
 		 * The only record we have of the real-time age of a
 		 * process, regardless of execs it's done, is start_time.
 		 * All the past CPU time is accumulated in signal_struct
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1998,6 +1998,13 @@ void posixtimer_send_sigqueue(struct k_i
 		return;
 
 	/*
+	 * If the signal is targeted at a specific thread, validate with sighand
+	 * lock held that the thread is not exiting.
+	 */
+	if (unlikely(tmr->it_pid_type == PIDTYPE_PID  && t->flags & PF_EXITING))
+		goto unlock;
+
+	/*
 	 * Update @tmr::sigqueue_seq for posix timer signals with sighand
 	 * locked to prevent a race against dequeue_signal().
 	 */
@@ -2088,6 +2095,7 @@ void posixtimer_send_sigqueue(struct k_i
 	result = TRACE_SIGNAL_DELIVERED;
 out:
 	trace_signal_generate(sig, &q->info, t, tmr->it_pid_type != PIDTYPE_PID, result);
+unlock:
 	unlock_task_sighand(t, &flags);
 }