From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:54245 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751453AbbATCxI convert rfc822-to-8bit (ORCPT ); Mon, 19 Jan 2015 21:53:08 -0500 Message-ID: <54BDC311.1050004@cn.fujitsu.com> Date: Tue, 20 Jan 2015 10:53:05 +0800 From: Qu Wenruo MIME-Version: 1.0 To: , , "miaoxie@huawei.com >> Miao Xie" Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock. References: <1421653361-18630-1-git-send-email-quwenruo@cn.fujitsu.com> <20150119140640.GB13289@twin.jikos.cz> <54BDC2C8.2090509@cn.fujitsu.com> In-Reply-To: <54BDC2C8.2090509@cn.fujitsu.com> Content-Type: text/plain; charset="utf-8"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: Add CC to Miao Xie -------- Original Message -------- Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock. From: Qu Wenruo To: dsterba@suse.cz, linux-btrfs@vger.kernel.org, Miao Xie Date: 2015年01月20日 10:51 > > -------- Original Message -------- > Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on > frozen fs to avoid deadlock. > From: David Sterba > To: Qu Wenruo > Date: 2015年01月19日 22:06 >> On Mon, Jan 19, 2015 at 03:42:41PM +0800, Qu Wenruo wrote: >>> The fix is to check if the fs is frozen, if the fs is frozen, just >>> return and waiting for the next transaction. >>> >>> --- a/fs/btrfs/super.c >>> +++ b/fs/btrfs/super.c >>> @@ -1000,6 +1000,14 @@ int btrfs_sync_fs(struct super_block *sb, int >>> wait) >>> */ >>> if (fs_info->pending_changes == 0) >>> return 0; >>> + /* >>> + * Test if the fs is frozen, or start_trasaction >>> + * will deadlock on itself. >>> + */ >>> + if (__sb_start_write(sb, SB_FREEZE_FS, false)) >>> + __sb_end_write(sb, SB_FREEZE_FS); >>> + else >>> + return 0; >> I'm not sure this is the right fix. We should use either >> mnt_want_write_file or sb_start_write around the start/commit functions. >> The fs may be frozen already, but we also have to catch transition to >> that state, or RO remount. > But the deadlock between s_umount and frozen level is a larger problem... > > Even Miao mentioned that we can start a transaction in btrfs_freeze(), > but there is still possibility that > we try to change the feature of the frozen btrfs and do sync, again > the deadlock will happen. > Although handling in btrfs_freeze() is also needed, but can't resolve > all the problem. > > IMHO the fix is still needed, or at least as a workaround until we > find a real root solution for it > (If nobody want to revert the patchset) > > BTW, what about put the pending changes to a workqueue? If we don't > start transaction under > s_umount context like sync_fs() > > Thanks, > Qu >> >> Also, returning 0 is not right, the ioctl actually skipped the expected >> work. >> >>> trans = btrfs_start_transaction(root, 0); >>> } else { >>> return PTR_ERR(trans); >