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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_PASS,T_DKIMWL_WL_HIGH 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 0819CC4646D for ; Sat, 11 Aug 2018 11:51:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B4EB721C16 for ; Sat, 11 Aug 2018 11:51:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="doY9n+Ei" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B4EB721C16 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org 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 S1727437AbeHKOZO (ORCPT ); Sat, 11 Aug 2018 10:25:14 -0400 Received: from mail.kernel.org ([198.145.29.99]:48808 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727266AbeHKOZO (ORCPT ); Sat, 11 Aug 2018 10:25:14 -0400 Received: from tleilax.poochiereds.net (cpe-71-70-156-158.nc.res.rr.com [71.70.156.158]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9303321B32; Sat, 11 Aug 2018 11:51:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1533988276; bh=lB1fxtSIz+OnOJG6BvD9ZmhnKWpENUR/Ndl5n0kj24s=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=doY9n+EizSTcTTlB6ZQr03bYh/XExzeahE/v/WDamHzBHVEakpL86p2DtnV6onr/u 16me+Eh66pdRxylPSfqF+egEWAVtqaSZXAD8glnqkFM1cDXcN2gfmHGtEtUpnP7ypx 4jgxw9wEEc5sKLNFszivunt4k4H+5cDVa2/hj3fE= Message-ID: Subject: Re: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups From: Jeff Layton To: "J. Bruce Fields" , NeilBrown Cc: Alexander Viro , Martin Wilck , linux-fsdevel@vger.kernel.org, Frank Filz , linux-kernel@vger.kernel.org Date: Sat, 11 Aug 2018 07:51:13 -0400 In-Reply-To: <20180810002922.GA3915@fieldses.org> References: <153378012255.1220.6754153662007899557.stgit@noble> <20180809173245.GM23873@fieldses.org> <87lg9frxyc.fsf@notabene.neil.brown.name> <20180810002922.GA3915@fieldses.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5 (3.28.5-1.fc28) 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 Thu, 2018-08-09 at 20:29 -0400, J. Bruce Fields wrote: > On Fri, Aug 10, 2018 at 08:12:43AM +1000, NeilBrown wrote: > > On Thu, Aug 09 2018, J. Bruce Fields wrote: > > > > > I think there's also a problem with multiple tasks sharing the same > > > lock owner. > > > > > > So, all locks are exclusive locks for the same range. We have four > > > tasks. Tasks 1 and 4 share the same owner, the others' owners are > > > distinct. > > > > > > - Task 1 gets a lock. > > > - Task 2 gets a conflicting lock. > > > - Task 3 gets another conflicting lock. So now we the tree is > > > 3->2->1. > > > - Task 1's lock is released. > > > - Before task 2 is scheduled, task 4 acquires a new lock. > > > - Task 2 waits on task 4's lock, we now have > > > 3->2->4. > > > > > > Task 3 shouldn't be waiting--the lock it's requesting has the same owner > > > as the lock task 4 holds--but we fail to wake up task 3. > > > > So task 1 and task 4 are threads in the one process - OK. > > Tasks 2 and 3 are threads in two other processes. > > > > So 2 and 3 conflict with either 1 or 4 equally - why should task 3 be > > woken? > > > > I suspect you got the numbers bit mixed up, > > Whoops. > > > but in any case, the "conflict()" function that is passed around takes > > ownership into account when assessing if one lock conflicts with > > another. > > Right, I know, but, let me try again: > > All locks are exclusive locks for the same range. Only tasks 3 and 4 > share the the same owner. > > - Task 1 gets a lock. > - Task 2 requests a conflicting lock, so we have 2->1. > - Task 3 requests a conflicting lock, so we have 3->2->1. > - Task 1 unlocks. We wake up task 2, but it isn't scheduled yet. > - Task 4 gets a new lock. > - Task 2 runs, discovers the conflict, and waits. Now we have: > 3->2->4. > > There is no conflict between the lock 3 requested and the lock 4 holds, > but 3 is not woken up. > > This is another version of the first problem: there's information we > need (the owners of the waiting locks in the tree) that we can't > determine just from looking at the root of the tree. > > I'm not sure what to do about that. > Is this still a problem in the v2 set? wake_non_conflicts walks the whole tree of requests that were blocked on it, so a. After task 2 discovers the conflict, it should wake any of its children that don't conflict. So in that last step, task 3 would be awoken before task 2 goes back to sleep. -- Jeff Layton