From nobody Sun Apr 19 20:31:26 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E30FC433EF for ; Mon, 27 Jun 2022 12:04:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239687AbiF0MEs (ORCPT ); Mon, 27 Jun 2022 08:04:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41660 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235473AbiF0MEf (ORCPT ); Mon, 27 Jun 2022 08:04:35 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4D5151263F for ; Mon, 27 Jun 2022 05:00:30 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C9527614FA for ; Mon, 27 Jun 2022 12:00:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 515CAC3411D; Mon, 27 Jun 2022 12:00:28 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="GBOW8G4J" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1656331226; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=ycbBTZciPg+5lBq40A1ayaOu2m5A5HL4W3lQwpUVo0M=; b=GBOW8G4JScRqESGRtEueEKDWk4iC8rLAGJEYhSRFbMOv3zC0aymEWKLVMQSQMfsaLIWQwb hhLLeyqmzIS+SIoNr9rO2EBmPETM0VBeSjz5M8g7JpPiySrizAs6/DFvuFxjRzmFEGkXWr /5EEccsGsNZukOi9SfHICYEVdXfewao= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 3bb9b886 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Mon, 27 Jun 2022 12:00:26 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, ebiederm@xmission.com Cc: "Jason A. Donenfeld" , =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= , Kalle Valo , Johannes Berg Subject: [PATCH] signal: break out of wait loops on kthread_stop() Date: Mon, 27 Jun 2022 14:00:20 +0200 Message-Id: <20220627120020.608117-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I was recently surprised to learn that msleep_interruptible(), wait_for_completion_interruptible_timeout(), and related functions simply hung when I called kthread_stop() on kthreads using them. The solution to fixing the case with msleep_interruptible() was more simply to move to schedule_timeout_interruptible(). Why? The reason is that msleep_interruptible(), and many functions just like it, has a loop like this: while (timeout && !signal_pending(current)) timeout =3D schedule_timeout_interruptible(timeout); The call to kthread_stop() woke up the thread, so schedule_timeout_ interruptible() returned early, but because signal_pending() returned true, it went back into another timeout, which was never woken up. This wait loop pattern is common to various pieces of code, and I suspect that subtle misuse in a kthread that caused a deadlock in the code I looked at last week is also found elsewhere. So this commit causes signal_pending() to return true when kthread_stop() is called. This is already what's done for TIF_NOTIFY_SIGNAL, for these same purposes of breaking out of wait loops, so a similar KTHREAD_SHOULD_STOP check isn't too much different. Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Eric W. Biederman Cc: Toke H=C3=B8iland-J=C3=B8rgensen Cc: Kalle Valo Cc: Johannes Berg Signed-off-by: Jason A. Donenfeld --- include/linux/kthread.h | 1 + include/linux/sched/signal.h | 9 +++++++++ kernel/kthread.c | 6 ++++++ 3 files changed, 16 insertions(+) diff --git a/include/linux/kthread.h b/include/linux/kthread.h index 30e5bec81d2b..7061dde23237 100644 --- a/include/linux/kthread.h +++ b/include/linux/kthread.h @@ -87,6 +87,7 @@ void kthread_bind(struct task_struct *k, unsigned int cpu= ); void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask); int kthread_stop(struct task_struct *k); bool kthread_should_stop(void); +bool __kthread_should_stop(struct task_struct *k); bool kthread_should_park(void); bool __kthread_should_park(struct task_struct *k); bool kthread_freezable_should_stop(bool *was_frozen); diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index cafbe03eed01..08700c65b806 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -11,6 +11,7 @@ #include #include #include +#include #include =20 /* @@ -397,6 +398,14 @@ static inline int signal_pending(struct task_struct *p) */ if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL))) return 1; + + /* + * Likewise, KTHREAD_SHOULD_STOP isn't really a signal, but it also + * requires the same behavior, lest wait loops go forever. + */ + if (unlikely(__kthread_should_stop(p))) + return 1; + return task_sigpending(p); } =20 diff --git a/kernel/kthread.c b/kernel/kthread.c index 3c677918d8f2..7e0743330cd4 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -145,6 +145,12 @@ void free_kthread_struct(struct task_struct *k) kfree(kthread); } =20 +bool __kthread_should_stop(struct task_struct *k) +{ + return (k->flags & PF_KTHREAD) && + test_bit(KTHREAD_SHOULD_STOP, &to_kthread(k)->flags); +} + /** * kthread_should_stop - should this kthread return now? * --=20 2.35.1