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=-9.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 82175C6786C for ; Fri, 14 Dec 2018 12:28:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 47D1720892 for ; Fri, 14 Dec 2018 12:28:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1544790500; bh=O4f0pEqHfjd5cTdBjwTNeaVyB3HGIGQZ72daYWTOzB8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=mZIKMnbLhzryGl3ployoO5QcuH1vikpXDdYcTxymLQjED2v4SztzmvT0Z143JnDZB mEpzLZCjqsNPDHZyVOmPZE+KWigCtLN45Db+6Yc/udIBf6nNKTM5Gxpw/oRPkG1+dm n+VISzX7XIKXtmqU25WyvxiZw9Gos93H/ns+Mpxc= DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 47D1720892 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linuxfoundation.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 S1731902AbeLNM2T (ORCPT ); Fri, 14 Dec 2018 07:28:19 -0500 Received: from mail.kernel.org ([198.145.29.99]:57314 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731684AbeLNMKy (ORCPT ); Fri, 14 Dec 2018 07:10:54 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (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 9F9EC2147C; Fri, 14 Dec 2018 12:10:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1544789454; bh=O4f0pEqHfjd5cTdBjwTNeaVyB3HGIGQZ72daYWTOzB8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=yAF5CqmdBasjD+IRKvGkamU8rbGcF8X+3yh3tCVTMSxA5AwgscB1/cwiqJDNEYv+7 /Eo1fBPtofMgnTgTtaYLIdyNMcu2l2Tsxs9lOg70tOgpTksgygdO8eHHCB742O9uIC NPhxL05gczebHw3F3CGghDTQV8T7YsxNOQKZ5jag= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, NeilBrown , David Howells , Sasha Levin Subject: [PATCH 4.14 69/89] fscache: fix race between enablement and dropping of object Date: Fri, 14 Dec 2018 13:00:22 +0100 Message-Id: <20181214115733.082397281@linuxfoundation.org> X-Mailer: git-send-email 2.20.0 In-Reply-To: <20181214115729.658859279@linuxfoundation.org> References: <20181214115729.658859279@linuxfoundation.org> User-Agent: quilt/0.65 X-stable: review X-Patchwork-Hint: ignore MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 4.14-stable review patch. If anyone has any objections, please let me know. ------------------ [ Upstream commit c5a94f434c82529afda290df3235e4d85873c5b4 ] 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(). At this time, ->backing_objects was empty, which would normaly prevent __fscache_read_or_alloc_page() from getting to the point of waiting. This implies that ->backing_objects was cleared *after* __fscache_read_or_alloc_page was was entered. When an object is "killed" and then "dropped", FSCACHE_COOKIE_LOOKING_UP is cleared in fscache_lookup_failure(), then KILL_OBJECT and DROP_OBJECT are "called" and only in DROP_OBJECT is ->backing_objects cleared. This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP and __fscache_read_or_alloc_page() can start waiting, before ->backing_objects is cleared 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. Customer which reported the hang, also report that the hang cannot be reproduced with this fix. The backtrace for the blocked process looked like: PID: 29360 TASK: ffff881ff2ac0f80 CPU: 3 COMMAND: "zsh" #0 [ffff881ff43efbf8] schedule at ffffffff815e56f1 #1 [ffff881ff43efc58] bit_wait at ffffffff815e64ed #2 [ffff881ff43efc68] __wait_on_bit at ffffffff815e61b8 #3 [ffff881ff43efca0] out_of_line_wait_on_bit at ffffffff815e625e #4 [ffff881ff43efd08] fscache_wait_for_deferred_lookup at ffffffffa04f2e8f [fscache] #5 [ffff881ff43efd18] __fscache_read_or_alloc_page at ffffffffa04f2ffe [fscache] #6 [ffff881ff43efd58] __nfs_readpage_from_fscache at ffffffffa0679668 [nfs] #7 [ffff881ff43efd78] nfs_readpage at ffffffffa067092b [nfs] #8 [ffff881ff43efda0] generic_file_read_iter at ffffffff81187a73 #9 [ffff881ff43efe50] nfs_file_read at ffffffffa066544b [nfs] #10 [ffff881ff43efe70] __vfs_read at ffffffff811fc756 #11 [ffff881ff43efee8] vfs_read at ffffffff811fccfa #12 [ffff881ff43eff18] sys_read at ffffffff811fda62 #13 [ffff881ff43eff50] entry_SYSCALL_64_fastpath at ffffffff815e986e Signed-off-by: NeilBrown Signed-off-by: David Howells Signed-off-by: Sasha Levin --- fs/fscache/object.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/fscache/object.c b/fs/fscache/object.c index 7a182c87f378..ab1d7f35f6c2 100644 --- a/fs/fscache/object.c +++ b/fs/fscache/object.c @@ -715,6 +715,9 @@ static const struct fscache_state *fscache_drop_object(struct fscache_object *ob if (awaken) wake_up_bit(&cookie->flags, FSCACHE_COOKIE_INVALIDATING); + if (test_and_clear_bit(FSCACHE_COOKIE_LOOKING_UP, &cookie->flags)) + wake_up_bit(&cookie->flags, FSCACHE_COOKIE_LOOKING_UP); + /* Prevent a race with our last child, which has to signal EV_CLEARED * before dropping our spinlock. -- 2.19.1