From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 27817C433B4 for ; Mon, 19 Apr 2021 19:58:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D57F861369 for ; Mon, 19 Apr 2021 19:58:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241951AbhDST7G (ORCPT ); Mon, 19 Apr 2021 15:59:06 -0400 Received: from foss.arm.com ([217.140.110.172]:49324 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235788AbhDST7F (ORCPT ); Mon, 19 Apr 2021 15:59:05 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 959521435; Mon, 19 Apr 2021 12:58:34 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 241F33F792; Mon, 19 Apr 2021 12:58:33 -0700 (PDT) From: Valentin Schneider To: Peter Zijlstra Cc: syzbot , bp@alien8.de, dwmw@amazon.co.uk, hpa@zytor.com, linux-kernel@vger.kernel.org, luto@kernel.org, mingo@redhat.com, syzkaller-bugs@googlegroups.com, tglx@linutronix.de, x86@kernel.org Subject: Re: [syzbot] WARNING in kthread_is_per_cpu In-Reply-To: <20210419184553.GA26214@worktop.programming.kicks-ass.net> References: <000000000000a61f7705c050e601@google.com> <87im4ilddh.mognet@arm.com> <20210419184553.GA26214@worktop.programming.kicks-ass.net> Date: Mon, 19 Apr 2021 20:58:26 +0100 Message-ID: <874kg2kpwd.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/04/21 20:45, Peter Zijlstra wrote: > On Mon, Apr 19, 2021 at 12:31:22PM +0100, Valentin Schneider wrote: > >> if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) >> `\ >> to_kthread(p); >> `\ >> WARN_ON(!(p->flags & PF_KTHREAD)); >> >> ... Huh? > > Something like so perhaps? > Looks about right, IIUC the key being: p->flags & PF_KTHREAD + p->set_child_tid => the struct kthread is persistent p->flags & PF_KTHREAD => you may or may not have a struct kthread (see kernel/umh.c kernel_thread() uses). PF_KTHREAD isn't even guaranteed to persist (begin_new_exec()), which seems to be what the syzbot hit. I'd be happy to see is_per_cpu_kthread() die, but that's somewhat orthogonal to this here. For now, this does need the tiny extra below. While we're at it, does free_kthread_struct() want the __to_kthread() treatment as well? The other to_kthread() callsites looked like they only made sense with a "proper" kthread anyway. --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 49636a49843f..8b470c2d5680 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7612,7 +7612,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) return 0; /* Disregard pcpu kthreads; they are where they need to be. */ - if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) + if (kthread_is_per_cpu(p)) return 0; if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { > diff --git a/kernel/kthread.c b/kernel/kthread.c > index 1578973c5740..eeba40df61ac 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -78,6 +78,14 @@ static inline void set_kthread_struct(void *kthread) > current->set_child_tid = (__force void __user *)kthread; > } > > +static inline struct kthread *__to_kthread(struct task_struct *k) > +{ > + void *kthread = (__force void *)k->set_child_tid; > + if (kthread && !(k->flags & PF_KTHREAD)) > + kthread = NULL; > + return kthread; > +} > + > static inline struct kthread *to_kthread(struct task_struct *k) > { > WARN_ON(!(k->flags & PF_KTHREAD)); > @@ -516,7 +524,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu) > > bool kthread_is_per_cpu(struct task_struct *k) > { > - struct kthread *kthread = to_kthread(k); > + struct kthread *kthread = __to_kthread(k); > if (!kthread) > return false; > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 3384ea74cad4..dc6311bd6986 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -7658,7 +7658,7 @@ static void balance_push(struct rq *rq) > * histerical raisins. > */ > if (rq->idle == push_task || > - ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) || > + kthread_is_per_cpu(push_task) || > is_migration_disabled(push_task)) { > > /*