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=-8.8 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 C2CA8C004D3 for ; Mon, 22 Oct 2018 18:56:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA9DA20651 for ; Mon, 22 Oct 2018 18:56:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=toxicpanda-com.20150623.gappssmtp.com header.i=@toxicpanda-com.20150623.gappssmtp.com header.b="CQXfqBiN" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AA9DA20651 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=toxicpanda.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-btrfs-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728763AbeJWDPn (ORCPT ); Mon, 22 Oct 2018 23:15:43 -0400 Received: from mail-qk1-f194.google.com ([209.85.222.194]:42634 "EHLO mail-qk1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728139AbeJWDPm (ORCPT ); Mon, 22 Oct 2018 23:15:42 -0400 Received: by mail-qk1-f194.google.com with SMTP id u20-v6so11475936qkk.9 for ; Mon, 22 Oct 2018 11:56:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=toxicpanda-com.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=h3UF+Kx59VbBcvDJIBJKdXYpMBpxjMe4j4XgxyIby7E=; b=CQXfqBiNuca7AKISDK9gyG0L0LFctOdOXm6l//8TQPIjObYWEyei4yEdLmlRHbGyyf 1MgCqZoUNcSnOdqb+LgC5zK+T5gsQ4xFvLpa1MbCBR8xJA4izWMuuR2X/ii8ZIqHdIYH r5CZDiK7lZBsuPN9zD+hgbAxbhW5boT3OxrgHrNJBF/W0YpDv5UKsEkREejq6BPvyV7Y Qew0aXJKzGjPc18VWDkKwHp7u7mKYNVqI0l67lW1wFsDn35YVFtvCcDVBQlLhgaaVTW8 QsPoGiG1Ig7jQBkvO2OVaSLTkxGNPBQyILD5I33lfPf5tHL6ayyOPsNLZFvEo++ZoSBH 0IAQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=h3UF+Kx59VbBcvDJIBJKdXYpMBpxjMe4j4XgxyIby7E=; b=fUob1TUHTeuXaqs+01TqcT1ZLY1WJuVamGwrdl2gKOR3bLeMFdNWBgybZNRBiPxJ4+ PmJRkH/YGs4LpQM7bLj8hQn5ayIxqXMMWcBULil/E4Gn4l5mAvHxXod4REz9j+1ef3Q6 fbOkcLufoNVFyZuUNi69kvkrfbVO+b4jqlqm8fmGaHmo6nHjtfDbm67E6FW4bOCauvrZ g6wJsdecavn9IcsUdUyQZZ9vAxjb4CgQQ64QdDs2+IJlqs3lx+q7uERyn3uyKb6zAvaj EFLo+UXTK4SCMp85c9G/nFgCEJuRf3iQ7u0IX9epO9VR/mHi/W3jyVpUpTk6OQEsFNyP hyrw== X-Gm-Message-State: ABuFfoihZEGvcMmkOKBuuQbrrOKQzTD0hx02gR1/RDTJl2eNiRdsNHTe ZuTs7P+IqN09oZXFFsfCbAw4sw== X-Google-Smtp-Source: ACcGV62H0ZqD2a87mX6P30+GMssvoE2PR/Ku68xqlGnHlK2Ye9NSQuyUDHu9vKhixxNaX4bN0LymvA== X-Received: by 2002:a37:655:: with SMTP id 82-v6mr43563815qkg.223.1540234559365; Mon, 22 Oct 2018 11:55:59 -0700 (PDT) Received: from localhost ([107.15.81.208]) by smtp.gmail.com with ESMTPSA id p6-v6sm4432082qtc.61.2018.10.22.11.55.58 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 22 Oct 2018 11:55:58 -0700 (PDT) Date: Mon, 22 Oct 2018 14:55:57 -0400 From: Josef Bacik To: fdmanana@kernel.org Cc: linux-btrfs@vger.kernel.org, josef@toxicpanda.com, Filipe Manana Subject: Re: [PATCH v2] Btrfs: fix deadlock on tree root leaf when finding free extent Message-ID: <20181022185556.2bfuoijklvsdyzx6@destiny> References: <20181022090946.1150-1-fdmanana@kernel.org> <20181022184830.415-1-fdmanana@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181022184830.415-1-fdmanana@kernel.org> User-Agent: NeoMutt/20180716 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Mon, Oct 22, 2018 at 07:48:30PM +0100, fdmanana@kernel.org wrote: > From: Filipe Manana > > When we are writing out a free space cache, during the transaction commit > phase, we can end up in a deadlock which results in a stack trace like the > following: > > schedule+0x28/0x80 > btrfs_tree_read_lock+0x8e/0x120 [btrfs] > ? finish_wait+0x80/0x80 > btrfs_read_lock_root_node+0x2f/0x40 [btrfs] > btrfs_search_slot+0xf6/0x9f0 [btrfs] > ? evict_refill_and_join+0xd0/0xd0 [btrfs] > ? inode_insert5+0x119/0x190 > btrfs_lookup_inode+0x3a/0xc0 [btrfs] > ? kmem_cache_alloc+0x166/0x1d0 > btrfs_iget+0x113/0x690 [btrfs] > __lookup_free_space_inode+0xd8/0x150 [btrfs] > lookup_free_space_inode+0x5b/0xb0 [btrfs] > load_free_space_cache+0x7c/0x170 [btrfs] > ? cache_block_group+0x72/0x3b0 [btrfs] > cache_block_group+0x1b3/0x3b0 [btrfs] > ? finish_wait+0x80/0x80 > find_free_extent+0x799/0x1010 [btrfs] > btrfs_reserve_extent+0x9b/0x180 [btrfs] > btrfs_alloc_tree_block+0x1b3/0x4f0 [btrfs] > __btrfs_cow_block+0x11d/0x500 [btrfs] > btrfs_cow_block+0xdc/0x180 [btrfs] > btrfs_search_slot+0x3bd/0x9f0 [btrfs] > btrfs_lookup_inode+0x3a/0xc0 [btrfs] > ? kmem_cache_alloc+0x166/0x1d0 > btrfs_update_inode_item+0x46/0x100 [btrfs] > cache_save_setup+0xe4/0x3a0 [btrfs] > btrfs_start_dirty_block_groups+0x1be/0x480 [btrfs] > btrfs_commit_transaction+0xcb/0x8b0 [btrfs] > > At cache_save_setup() we need to update the inode item of a block group's > cache which is located in the tree root (fs_info->tree_root), which means > that it may result in COWing a leaf from that tree. If that happens we > need to find a free metadata extent and while looking for one, if we find > a block group which was not cached yet we attempt to load its cache by > calling cache_block_group(). However this function will try to load the > inode of the free space cache, which requires finding the matching inode > item in the tree root - if that inode item is located in the same leaf as > the inode item of the space cache we are updating at cache_save_setup(), > we end up in a deadlock, since we try to obtain a read lock on the same > extent buffer that we previously write locked. > > So fix this by skipping the loading of free space caches of any block > groups that are not yet cached (rare cases) if we are COWing an extent > buffer from the root tree and space caching is enabled (-o space_cache > mount option). This is a rare case and its downside is failure to > find a free extent (return -ENOSPC) when all the already cached block > groups have no free extents. > > Reported-by: Andrew Nelson > Link: https://lore.kernel.org/linux-btrfs/CAPTELenq9x5KOWuQ+fa7h1r3nsJG8vyiTH8+ifjURc_duHh2Wg@mail.gmail.com/ > Fixes: 9d66e233c704 ("Btrfs: load free space cache if it exists") > Tested-by: Andrew Nelson > Signed-off-by: Filipe Manana > --- > > V2: Made the solution more generic, since the problem could happen in any > path COWing an extent buffer from the root tree. > > Applies on top of a previous patch titled: > > "Btrfs: fix deadlock when writing out free space caches" > > fs/btrfs/ctree.c | 4 ++++ > fs/btrfs/ctree.h | 3 +++ > fs/btrfs/disk-io.c | 2 ++ > fs/btrfs/extent-tree.c | 15 ++++++++++++++- > 4 files changed, 23 insertions(+), 1 deletion(-) > > diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c > index 089b46c4d97f..646aafda55a3 100644 > --- a/fs/btrfs/ctree.c > +++ b/fs/btrfs/ctree.c > @@ -1065,10 +1065,14 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans, > root == fs_info->chunk_root || > root == fs_info->dev_root) > trans->can_flush_pending_bgs = false; > + else if (root == fs_info->tree_root) > + atomic_inc(&fs_info->tree_root_cows); > > cow = btrfs_alloc_tree_block(trans, root, parent_start, > root->root_key.objectid, &disk_key, level, > search_start, empty_size); > + if (root == fs_info->tree_root) > + atomic_dec(&fs_info->tree_root_cows); Do we need this though? Our root should be the root we're cow'ing the block for, and it should be passed all the way down to find_free_extent properly, so we really should be able to just do if (root == fs_info->tree_root) and not add all this stuff. Not to mention this will race with anybody else doing stuff, so if another thread that isn't actually touching the tree_root it would skip caching a block group when it's completely ok for that thread to do it. Thanks, Josef