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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,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 3AB1AC433E6 for ; Tue, 12 Jan 2021 09:19:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D4F2122DFA for ; Tue, 12 Jan 2021 09:19:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390634AbhALJSf (ORCPT ); Tue, 12 Jan 2021 04:18:35 -0500 Received: from mx2.suse.de ([195.135.220.15]:40550 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2392544AbhALJSd (ORCPT ); Tue, 12 Jan 2021 04:18:33 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610443066; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=XflxbYa9LUQjLUev+xvZhTvWAFz42QmEpPdg8UIk/j0=; b=I+Aj3juG3UMMjorj4PVReAPEsOk3haoGI4qhfh0WGkEbPY8bQsyRDaOlIP2H5NwD63XWyU +uw+lryST4nocwcBHQ3NsRKRwMtu274opccL7e7iYDXeQT7ufe+m2hxOoP5KgiYAcexeI+ ziZBulIVX4X0JpIfQDSYakOJrXZeE2Q= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CBEBFB7DE; Tue, 12 Jan 2021 09:17:46 +0000 (UTC) Subject: Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit To: dsterba@suse.cz, Nikolay Borisov , Josef Bacik , linux-btrfs@vger.kernel.org, kernel-team@fb.com References: <9e47b11bdfe5b4905fdaa81e952de2e2466c6335.1608319304.git.josef@toxicpanda.com> <20210108160109.GB6430@twin.jikos.cz> <52aef9a6-efc7-0820-7056-067e69c2a856@suse.com> <20210111215051.GH6430@twin.jikos.cz> From: Nikolay Borisov Autocrypt: addr=nborisov@suse.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210111215051.GH6430@twin.jikos.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 11.01.21 г. 23:50 ч., David Sterba wrote: > On Mon, Jan 11, 2021 at 10:33:42AM +0200, Nikolay Borisov wrote: >> On 8.01.21 г. 18:01 ч., David Sterba wrote: >>> On Fri, Dec 18, 2020 at 02:24:20PM -0500, Josef Bacik wrote: >>>> @@ -2043,23 +2043,22 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans) >>>> btrfs_trans_release_metadata(trans); >>>> trans->block_rsv = NULL; >>>> >>>> - /* make a pass through all the delayed refs we have so far >>>> - * any runnings procs may add more while we are here >>>> - */ >>>> - ret = btrfs_run_delayed_refs(trans, 0); >>>> - if (ret) { >>>> - btrfs_end_transaction(trans); >>>> - return ret; >>>> - } >>>> - >>>> - cur_trans = trans->transaction; >>>> - >>>> /* >>>> - * set the flushing flag so procs in this transaction have to >>>> - * start sending their work down. >>>> + * We only want one transaction commit doing the flushing so we do not >>>> + * waste a bunch of time on lock contention on the extent root node. >>>> */ >>>> - cur_trans->delayed_refs.flushing = 1; >>>> - smp_wmb(); >>> >>> This barrier obviously separates the flushing = 1 and the rest of the >>> code, now implemented as test_and_set_bit, which implies full barrier. >>> >>> However, hunk in btrfs_should_end_transaction removes the barrier and >>> I'm not sure whether this is correct: >>> >>> - smp_mb(); >>> if (cur_trans->state >= TRANS_STATE_COMMIT_START || >>> - cur_trans->delayed_refs.flushing) >>> + test_bit(BTRFS_DELAYED_REFS_FLUSHING, >>> + &cur_trans->delayed_refs.flags)) >>> return true; >>> >>> This is never called under locks so we don't have complete >>> synchronization of neither the transaction state nor the flushing bit. >>> btrfs_should_end_transaction is merely a hint and not called in critical >>> places so we could probably afford to keep it without a barrier, or keep >>> it with comment(s). >> >> I think the point is moot in this case, because the test_bit either sees >> the flag or it doesn't. It's not possible for the flag to be set AND >> should_end_transaction return false that would be gross violation of >> program correctness. > > So that's for the flushing part, but what about cur_trans->state? Looking at the code, the barrier was there to order the publishing of the delayed_ref.flushing (now replaced by the bit flag) against surrounding code. So independently of this patch, let's reason about trans state. In should_end_transaction it's read without holding any locks. (U) It's modified in btrfs_cleanup_transaction without holding the fs_info->trans_lock (U), but the STATE_ERROR flag is going to be set. set in cleanup_transaction under fs_info->trans_lock (L) set in btrfs_commit_trans to COMMIT_START under fs_info->trans_lock.(L) set in btrfs_commit_trans to COMMIT_DOING under fs_info->trans_lock.(L) set in btrfs_commit_trans to COMMIT_UNBLOCK under fs_info->trans_lock.(L) set in btrfs_commit_trans to COMMIT_COMPLETED without locks but at this point the transaction is finished and fs_info->running_trans is NULL (U but irrelevant). So by the looks of it we can have a concurrent READ race with a Write, due to reads not taking a lock. In this case what we want to ensure is we either see new or old state. I consulted with Will Deacon and he said that in such a case we'd want to annotate the accesses to ->state with (READ|WRITE)_ONCE so as to avoid a theoretical tear, in this case I don't think this could happen but I imagine at some point kcsan would flag such an access as racy (which it is). >