From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS17314 8.43.84.0/22 X-Spam-Status: No, score=-3.7 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RDNS_DYNAMIC,SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (ip-8-43-85-97.sourceware.org [8.43.85.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id BE2EF1F8C6 for ; Mon, 30 Aug 2021 11:27:55 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id AA215385782A for ; Mon, 30 Aug 2021 11:27:54 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org AA215385782A DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1630322874; bh=9FDAoYUWVvThyuZwctmc+BDmeBTYhn2sMBsoXQWEt0g=; h=To:Subject:References:Date:In-Reply-To:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=t0j8jo40k7hZjEyi+G0HtSsC0zoGRiYNwEliv4bBmOeYfBY+WeZYcELngK/w3eihT UcCTQ2rzFexTSYUclHNT4kWCGokJ/RW0ij4N8ym9jddYFUZQuy6OLdidmenDNCyvv9 JpztQLSvgZCVjMqmacgOTPVOYbc01wLkc4MaTHl4= Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 6F88D3858D35 for ; Mon, 30 Aug 2021 11:27:35 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 6F88D3858D35 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-561-JyComApFNwaZbNTnyzcRbA-1; Mon, 30 Aug 2021 07:27:03 -0400 X-MC-Unique: JyComApFNwaZbNTnyzcRbA-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id CE128801E72; Mon, 30 Aug 2021 11:27:02 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.39.194.140]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F1B0127C53; Mon, 30 Aug 2021 11:27:01 +0000 (UTC) To: Adhemerval Zanella Subject: Re: [PATCH v2 05/19] nptl: Move setxid flag out of cancelhandling References: <20210823195047.543237-1-adhemerval.zanella@linaro.org> <20210823195047.543237-6-adhemerval.zanella@linaro.org> <87tujc783g.fsf@oldenburg.str.redhat.com> <87o89k44gw.fsf@oldenburg.str.redhat.com> <9282d6d3-cbb5-7975-31c9-f4088c594777@linaro.org> Date: Mon, 30 Aug 2021 13:27:00 +0200 In-Reply-To: <9282d6d3-cbb5-7975-31c9-f4088c594777@linaro.org> (Adhemerval Zanella's message of "Thu, 26 Aug 2021 13:39:53 -0300") Message-ID: <87r1ebqikb.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 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain 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: , From: Florian Weimer via Libc-alpha Reply-To: Florian Weimer Cc: libc-alpha@sourceware.org Errors-To: libc-alpha-bounces+e=80x24.org@sourceware.org Sender: "Libc-alpha" * Adhemerval Zanella: > On 26/08/2021 12:21, Florian Weimer wrote: >> * Adhemerval Zanella: >> >>> On 26/08/2021 08:34, Florian Weimer wrote: >>>> * Adhemerval Zanella: >>>> >>>>> Now that the thread state is tracked by 'joinstate' field, there is no >>>>> need to keep the setxid flag within the 'cancelhandling'. It simplifies >>>>> the atomic code to set and reset it (since there is no need to handle >>>>> the thread state concurrent update). >>>> >>>> Some of this functionality reimplements the tidlock. >>>> >>>> There is some functionality around thread creation, but I do not know if >>>> the synchronization is entirely correct. This makes it rather difficult >>>> to review changes. >>>> >>>> It's also not clear to me if we need to do anything to prevent early >>>> setxid calls: >>>> >>>>> /* Don't allow setxid until cloned. */ >>>> >>>> I suspect this should just look at the TID instead. We create the >>>> thread with all signals blocked, so once there is a TID (and the thread >>>> has not exited), it is safe to send the signal. >>> >>> I agree that we might improve the setxid code, however what I intended >>> with this changes is to keep its semantic as-is and just decouple it >>> from cancelhandling. I might revise it on a later patch to follow your >>> suggestions. >> >> I wonder if there is a new race here: >> >> @@ -109,41 +100,23 @@ setxid_mark_thread (struct xid_command *cmdp, struct pthread *t) >> /* Don't let the thread exit before the setxid handler runs. */ >> t->setxid_futex = 0; >> >> + /* If thread is exiting right now, ignore it. */ >> + if (atomic_load_acquire (&t->joinstate) == THREAD_STATE_EXITING) >> + return; >> >> + /* Release the futex if there is no other setxid in progress. */ >> + if (atomic_exchange_acquire (&t->setxid_flag, 1) == 0) >> + { >> + t->setxid_futex = 1; >> + futex_wake (&t->setxid_futex, 1, FUTEX_PRIVATE); >> } >> >> That is, if the setxid-sending thread can now wait for a futex wake that >> never comes. > > Wouldn't be handled by setxid_unmark_thread, which wakes the futex > unconditionally? But that code only runs after the loop around signalled has concluded. That's why I'm worried. Thanks, Florian