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 48C52C38142 for ; Mon, 23 Jan 2023 16:50:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232621AbjAWQur (ORCPT ); Mon, 23 Jan 2023 11:50:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55026 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231623AbjAWQuq (ORCPT ); Mon, 23 Jan 2023 11:50:46 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C98AB298C1 for ; Mon, 23 Jan 2023 08:49:58 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1674492598; 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: in-reply-to:in-reply-to:references:references; bh=lehvOBS49+YlatZIl8TexmyosPGVnClShITP1EubXVM=; b=ZSIDgEm1wNehS0PR14XR+PU2pE+sdT9lTJKDOXAzG8zhJPy3GH7+ktsn9Rhm19/cNfHDc6 xP7W12Rhh0HrYryRQ/FK1UJ9md/+B2RWbbYvh1nvWDesOGMhKZbX2qBMsdJ0912/fe6xrI 6n8KjE3YB2dBoKmd9lwgIGl26NGA3Qk= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-466-fc1bisz2OYqcg2nNu2Y23w-1; Mon, 23 Jan 2023 11:49:53 -0500 X-MC-Unique: fc1bisz2OYqcg2nNu2Y23w-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id A70C7183B3C0; Mon, 23 Jan 2023 16:49:52 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (ovpn-192-224.brq.redhat.com [10.40.192.224]) by smtp.corp.redhat.com (Postfix) with SMTP id 5AB07140EBF4; Mon, 23 Jan 2023 16:49:47 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 23 Jan 2023 17:49:50 +0100 (CET) Date: Mon, 23 Jan 2023 17:49:44 +0100 From: Oleg Nesterov To: Wander Lairson Costa Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider , "Eric W. Biederman" , Stafford Horne , Kefeng Wang , Andrew Morton , Thomas Gleixner , Sebastian Andrzej Siewior , Andy Lutomirski , "Liam R. Howlett" , Fenghua Yu , Andrei Vagin , open list , Paul McKenney Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function Message-ID: <20230123164943.GC6268@redhat.com> References: <20230120150246.20797-1-wander@redhat.com> <20230120150246.20797-2-wander@redhat.com> <20230123163002.GB6268@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230123163002.GB6268@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/23, Oleg Nesterov wrote: > > On 01/20, Wander Lairson Costa wrote: > > > > +static inline void put_task_struct_atomic_safe(struct task_struct *task) > > +{ > > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) { > > + /* > > + * Decrement the refcount explicitly to avoid unnecessarily > > + * calling call_rcu. > > + */ > > + if (refcount_dec_and_test(&task->usage)) > > + /* > > + * under PREEMPT_RT, we can't call put_task_struct > > + * in atomic context because it will indirectly > > + * acquire sleeping locks. > > + */ > > + call_rcu(&task->rcu, __delayed_put_task_struct); > ^^^^^^^^^ > I am not sure the usage of task->rcu is safe... > > Suppose that, before __delayed_put_task_struct() is called by RCU, this task > does the last schedule and calls put_task_struct_rcu_user(). Ah, sorry, please forget, rcu_users != 0 implies task->usage != 0. > And, can't we simply turn put_task_struct() into something like > > put_task_struct(struct task_struct *t) > { > if (refcount_dec_and_test(&t->usage)) { > if (IS_ENABLED(CONFIG_PREEMPT_RT) > && (in_atomic() || irqs_disabled())) > call_rcu(...); > else > __put_task_struct(t); > } > } > > ? > > Oleg.