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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 D79B7C6778D for ; Tue, 11 Sep 2018 13:33:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 915922086A for ; Tue, 11 Sep 2018 13:33:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=arista.com header.i=@arista.com header.b="ACMsuGxB" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 915922086A Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=arista.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727748AbeIKScr (ORCPT ); Tue, 11 Sep 2018 14:32:47 -0400 Received: from mail-ed1-f66.google.com ([209.85.208.66]:38014 "EHLO mail-ed1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726622AbeIKScq (ORCPT ); Tue, 11 Sep 2018 14:32:46 -0400 Received: by mail-ed1-f66.google.com with SMTP id h33-v6so19237728edb.5 for ; Tue, 11 Sep 2018 06:33:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=arista.com; s=googlenew; h=message-id:subject:from:to:cc:date:in-reply-to:references :mime-version:content-transfer-encoding; bh=QUfvWbNmojkePg9NAe1H0MLdWfRQ5KsQXF5Oh04nS+k=; b=ACMsuGxBSs0CZ02qcKD4qwaUIErg0yypo+pj+vecQ7lnVIKuPNtQsRBp11ybEZpITJ YuPQsgfrpXzeaM00Ke1tKW+bGLvw2LXzq8vz2t8rz2EXnfnoylXB5v97/auVZ0CDeXsB d+N+a6T2cjAT+0B2bdrE4LamHTHZ53RwhVo6fZVi+neKw7IUaXjdWYUmYvPfc9JwdpYg OzuvaFbFj8qUtgM+ezzMihGblVFuz6QYAILaVqcQovr7r7FKHCPxtngJJrZ+pwuXmKql HRq1vvxEjmWQvG4psckkZXu0ckrFE0TxT5aVsoh49pFgjY2Eyae1CvosNb+rkwyKWsVS hZPQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:message-id:subject:from:to:cc:date:in-reply-to :references:mime-version:content-transfer-encoding; bh=QUfvWbNmojkePg9NAe1H0MLdWfRQ5KsQXF5Oh04nS+k=; b=btiZ2KUK/9hEiuQmOTXJhbvX/mo4Dt1SY0XhYwdQHapONVoQTXZpyPC/4ixxX12uO5 gBXLW5/WdXC94FiUM2kQQHzcTg9CESmBoptR2KjUa9TZ9VlmrrHY0CgP5MFTTe8AN8Kh To/97dgIMfBe2cDxkB1jXuI8sxJZdvoN+zDUaszhqLbBl6WHV4mfHZCYzsShOhTYVAg4 oxyx7YhL0ZG8FwRzADGtVgiza+2XhWRQ7oq4aw75a6uAk4Gb/ns9IqEuwPRAIs2HtUVz I8hJFx4AgIWc98ZqLsz3etpvAeM4IkGkMvgjhE7UNWJ+P5c8rhXNuXzwtstNR+EOdvr7 uZIQ== X-Gm-Message-State: APzg51A3pkij3nL/kp/bYSANyzjlU5P6bpQJxTNi9RTAB6w4bcKN8F3Q +kRonCmXRN30BWas+0T+xqr/vg== X-Google-Smtp-Source: ANB0VdbZbze2fRZY9ig51PiTNtYnDZYS2+KKKXuU9zOs5cDH3nHyx00WiE+T4xPVZToW1J7MPVh4sg== X-Received: by 2002:a50:a9e2:: with SMTP id n89-v6mr28282363edc.158.1536672804351; Tue, 11 Sep 2018 06:33:24 -0700 (PDT) Received: from dhcp.ire.aristanetworks.com ([217.173.96.166]) by smtp.gmail.com with ESMTPSA id y27-v6sm9895120edb.20.2018.09.11.06.33.22 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 11 Sep 2018 06:33:23 -0700 (PDT) Message-ID: <1536672802.2710.33.camel@arista.com> Subject: Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted down_read() From: Dmitry Safonov To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Dmitry Safonov <0x7f454c46@gmail.com>, Daniel Axtens , Dmitry Vyukov , Michael Neuling , Mikulas Patocka , Nathan March , Pasi =?ISO-8859-1?Q?K=E4rkk=E4inen?= , Peter Hurley , "Rong, Chen" , Sergey Senozhatsky , Tan Xiaojun , Tetsuo Handa , Greg Kroah-Hartman , Jiri Slaby Date: Tue, 11 Sep 2018 14:33:22 +0100 In-Reply-To: <1536670918.2710.29.camel@arista.com> References: <20180911014821.26286-1-dima@arista.com> <20180911014821.26286-7-dima@arista.com> <20180911120258.GC19234@hirez.programming.kicks-ass.net> <1536670918.2710.29.camel@arista.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.24.6 (3.24.6-1.fc26) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2018-09-11 at 14:01 +0100, Dmitry Safonov wrote: > On Tue, 2018-09-11 at 14:02 +0200, Peter Zijlstra wrote: > > On Tue, Sep 11, 2018 at 02:48:21AM +0100, Dmitry Safonov wrote: > > > It seems like when ldsem_down_read() fails with timeout, it > > > misses > > > update for sem->wait_readers. By that reason, when writer finally > > > releases write end of the semaphore __ldsem_wake_readers() does > > > adjust > > > sem->count with wrong value: > > > sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS) > > > > > > I.e, if update comes with 1 missed wait_readers decrement, sem- > > > > count > > > > > > will be 0x100000001 which means that there is active reader and > > > it'll > > > make any further writer to fail in acquiring the semaphore. > > > > > > It looks like, this is a dead-code, because ldsem_down_read() is > > > never > > > called with timeout different than MAX_SCHEDULE_TIMEOUT, so it > > > might be > > > worth to delete timeout parameter and error path fall-back.. > > > > You might want to think about ditching that ldsem thing entirely, > > and > > use a regular rwsem ? > > Yeah, but AFAICS, regular rwsem will need to have a timeout then (for > write). So, I thought fixing this pile would be simpler than adding > timeout and probably writer-priority to generic rwsem? > > And I guess, we still will need fixes for stable for the bugs here.. > > I expect that timeouts are ABI, while the gain of adding priority may > be measured. I'll give it a shot (adding timeout/priority for linux- > next) to rwsem if you say it's acceptable. Actually, priority looks quite simple: we can add writers in the head of wait_list and it probably may work. Timeout looks also not a rocket science. So, I can try to do that if you say it's acceptable (with the gain measures). After this can of worms that I need to fix regardless. -- Thanks, Dmitry