From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id 1C7DB385840A for ; Thu, 11 Nov 2021 13:54:16 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1C7DB385840A Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-36-qhz5sUMbMru9y-WCjVlCbg-1; Thu, 11 Nov 2021 08:54:14 -0500 X-MC-Unique: qhz5sUMbMru9y-WCjVlCbg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C49A28799F4; Thu, 11 Nov 2021 13:54:13 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.39.192.82]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B6E735DA61; Thu, 11 Nov 2021 13:54:12 +0000 (UTC) From: Florian Weimer To: Adhemerval Zanella Cc: Adhemerval Zanella via Libc-alpha , leonardo.macchia@gmail.com Subject: Re: [PATCH 1/3] inet: Fix getnameinfo (NI_NOFQDN) race condition (BZ#28566) References: <20211110185832.1931688-1-adhemerval.zanella@linaro.org> <20211110185832.1931688-2-adhemerval.zanella@linaro.org> <87r1bn150g.fsf@oldenburg.str.redhat.com> <27605b2b-cde0-9b1a-fc97-882d30e30b49@linaro.org> Date: Thu, 11 Nov 2021 14:54:10 +0100 In-Reply-To: <27605b2b-cde0-9b1a-fc97-882d30e30b49@linaro.org> (Adhemerval Zanella's message of "Thu, 11 Nov 2021 10:34:27 -0300") Message-ID: <87czn6yf0d.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Spam-Status: No, score=-12.7 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 11 Nov 2021 13:54:17 -0000 * Adhemerval Zanella: > On 11/11/2021 05:16, Florian Weimer wrote: >> * Adhemerval Zanella via Libc-alpha: >> >>> diff --git a/inet/getnameinfo.c b/inet/getnameinfo.c >>> index 8380d85783..58ebbb1154 100644 >>> --- a/inet/getnameinfo.c >>> +++ b/inet/getnameinfo.c >>> @@ -86,55 +86,75 @@ libc_freeres_ptr (static char *domain); >>> static char * >>> nrl_domainname (void) >>> { >>> + __libc_lock_define_initialized (static, lock); >>> + __libc_lock_lock (lock); >>> >>> + static bool not_first = false; >>> if (! not_first) >> >>> + done: >>> + scratch_buffer_free (&tmpbuf); >>> + not_first = true; >> >> This is missing the acquire/release pairing for the double-checked >> locking idiom. You can probably use the domain variable directly. > > But it is done now within the lock, different than current implementation > which does outside. I moved to be within the lock exactly to avoid the > double-checked locking idiom. Ah, sorry, I had missed that. > I think now that we might be moving to a more optimized lll_lock internally > using a acquire-load+CAS instead of just CAS we can get it without need > to code it explicitly. The double-checked locking idiom avoids the CAS after initialization. With the lll_lock change, an atomic read-modify-write operation still happens on the lock in all cases (prior to the eventual return to the caller). Thanks, Florian