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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 4861EC43441 for ; Wed, 28 Nov 2018 15:12:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 13D472081B for ; Wed, 28 Nov 2018 15:12:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 13D472081B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.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 S1728590AbeK2COa (ORCPT ); Wed, 28 Nov 2018 21:14:30 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36378 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727726AbeK2COa (ORCPT ); Wed, 28 Nov 2018 21:14:30 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 2E23530842AB; Wed, 28 Nov 2018 15:12:32 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-120-113.rdu2.redhat.com [10.10.120.113]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4406B1019624; Wed, 28 Nov 2018 15:12:31 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <87r2gd44hu.fsf@notabene.neil.brown.name> References: <87r2gd44hu.fsf@notabene.neil.brown.name> To: NeilBrown Cc: dhowells@redhat.com, linux-cachefs@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fscache: fix race between enablement and dropping of object MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <10284.1543417950.1@warthog.procyon.org.uk> Date: Wed, 28 Nov 2018 15:12:30 +0000 Message-ID: <10285.1543417950@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Wed, 28 Nov 2018 15:12:32 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org NeilBrown wrote: > It was observed that a process blocked indefintely in > __fscache_read_or_alloc_page(), waiting for FSCACHE_COOKIE_LOOKING_UP > to be cleared via fscache_wait_for_deferred_lookup(). Was there a backtrace for that? > This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP Attempting to enable the cookie again would have that effect. > There is some uncertainty in this analysis, but it seems to be fit the > observations. Adding the wake in this patch will be handled correctly > by __fscache_read_or_alloc_page(), as it checks if ->backing_objects > is empty again, after waiting. That should work. The read op should just then fail with -ENOBUFS, causing the netfs to issue the read over the network instead. I think there's a more elegant solution to be had, but it'll take some thought as to how to achieve it. David