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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 3165EC04AAF for ; Thu, 16 May 2019 14:29:25 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id B2D6820833 for ; Thu, 16 May 2019 14:29:24 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B2D6820833 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 454Yhf5MTnzDqcn for ; Fri, 17 May 2019 00:29:22 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=redhat.com (client-ip=209.132.183.28; helo=mx1.redhat.com; envelope-from=oleg@redhat.com; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=redhat.com Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 454YfG1HhKzDqc6 for ; Fri, 17 May 2019 00:27:18 +1000 (AEST) Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7E7E388E5D; Thu, 16 May 2019 14:27:11 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.159]) by smtp.corp.redhat.com (Postfix) with SMTP id 144FC341E2; Thu, 16 May 2019 14:27:01 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Thu, 16 May 2019 16:27:10 +0200 (CEST) Date: Thu, 16 May 2019 16:27:00 +0200 From: Oleg Nesterov To: Christian Brauner Subject: Re: [PATCH v1 1/2] pid: add pidfd_open() Message-ID: <20190516142659.GB22564@redhat.com> References: <20190516135944.7205-1-christian@brauner.io> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190516135944.7205-1-christian@brauner.io> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Thu, 16 May 2019 14:27:14 +0000 (UTC) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, linux-mips@vger.kernel.org, dhowells@redhat.com, joel@joelfernandes.org, linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org, linux-api@vger.kernel.org, elena.reshetova@intel.com, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, dancol@google.com, Geert Uytterhoeven , serge@hallyn.com, linux-xtensa@linux-xtensa.org, keescook@chromium.org, arnd@arndb.de, jannh@google.com, linux-m68k@lists.linux-m68k.org, viro@zeniv.linux.org.uk, luto@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org, cyphar@cyphar.com, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, luto@amacapital.net, ebiederm@xmission.com, linux-alpha@vger.kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 05/16, Christian Brauner wrote: > > With the introduction of pidfds through CLONE_PIDFD it is possible to > created pidfds at process creation time. Now I am wondering why do we need CLONE_PIDFD, you can just do pid = fork(); pidfd_open(pid); > +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags) > +{ > + int fd, ret; > + struct pid *p; > + struct task_struct *tsk; > + > + if (flags) > + return -EINVAL; > + > + if (pid <= 0) > + return -EINVAL; > + > + p = find_get_pid(pid); > + if (!p) > + return -ESRCH; > + > + ret = 0; > + rcu_read_lock(); > + /* > + * If this returns non-NULL the pid was used as a thread-group > + * leader. Note, we race with exec here: If it changes the > + * thread-group leader we might return the old leader. > + */ > + tsk = pid_task(p, PIDTYPE_TGID); > + if (!tsk) > + ret = -ESRCH; > + rcu_read_unlock(); > + > + fd = ret ?: pidfd_create(p); > + put_pid(p); > + return fd; > +} Looks correct, feel free to add Reviewed-by: Oleg Nesterov But why do we need task_struct *tsk? rcu_read_lock(); if (!pid_task(PIDTYPE_TGID)) ret = -ESRCH; rcu_read_unlock(); and in fact we do not even need rcu_read_lock(), we could do // shut up rcu_dereference_check() rcu_lock_acquire(&rcu_lock_map); if (!pid_task(PIDTYPE_TGID)) ret = -ESRCH; rcu_lock_release(&rcu_lock_map); Well... I won't insist, but the comment about the race with exec looks a bit confusing to me. It is true, but we do not care at all, we are not going to use the task_struct returned by pid_task(). Oleg.