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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 BACDFC2BA2B for ; Fri, 10 Apr 2020 16:30:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E71620769 for ; Fri, 10 Apr 2020 16:30:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726638AbgDJQaZ (ORCPT ); Fri, 10 Apr 2020 12:30:25 -0400 Received: from mx2.suse.de ([195.135.220.15]:52686 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726173AbgDJQaZ (ORCPT ); Fri, 10 Apr 2020 12:30:25 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 3D6DAABD6; Fri, 10 Apr 2020 16:30:23 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 3F351DA72D; Fri, 10 Apr 2020 18:29:46 +0200 (CEST) Date: Fri, 10 Apr 2020 18:29:46 +0200 From: David Sterba To: Nikolay Borisov Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root Message-ID: <20200410162946.GN5920@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Nikolay Borisov , linux-btrfs@vger.kernel.org References: <20200221131124.24105-1-nborisov@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200221131124.24105-1-nborisov@suse.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Fri, Feb 21, 2020 at 03:11:23PM +0200, Nikolay Borisov wrote: > __del_reloc_root is called from: > > a) Transaction commit via: > btrfs_transaction_commit > commit_fs_roots > btrfs_update_reloc_root > __del_reloc_root > > b) From the relocation thread with the following call chains: > > relocate_block_group > merge_reloc_roots > insert_dirty_subvol > btrfs_update_reloc_root > __del_reloc_root > > c) merge_reloc_roots > free_reloc_roots > __del_reloc_roots > > (The above call chain can called from btrfs_recover_relocation as well > but for the purpose of this fix this is irrelevant). > > The commont data structure that needs protecting is > fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list. > Turns out it's no needed to hold the trans_lock in __del_reloc_root > since consistency is already guaranteed by call chain b) above holding > a transaction while calling insert_dirty_subvol, meaning we cannot have > a concurrent transaction commit. For call chain c) above a snapshot of > the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and > free_reloc_roots is called on this local snapshot. > > Those invariants are sufficient to prevent racing calls to > __del_reloc_root alongside other users of the list, as such it's fine > to drop the lock acquisition. > > Signed-off-by: Nikolay Borisov > --- > fs/btrfs/relocation.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c > index 8076c340749f..e5cb64409f7c 100644 > --- a/fs/btrfs/relocation.c > +++ b/fs/btrfs/relocation.c > @@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root) > BUG_ON((struct btrfs_root *)node->data != root); > } > > - spin_lock(&fs_info->trans_lock); > list_del_init(&root->root_list); > - spin_unlock(&fs_info->trans_lock); This has been obsoleted by patch f44deb7442edf42abee6 ("btrfs: hold a ref on the root->reloc_root"), I think the locks are still needed but you may want take a look.