From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cantor2.suse.de ([195.135.220.15]:60799 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752872AbaDDNkq (ORCPT ); Fri, 4 Apr 2014 09:40:46 -0400 Date: Fri, 4 Apr 2014 15:40:44 +0200 From: David Sterba To: Liu Bo Cc: dsterba@suse.cz, linux-btrfs@vger.kernel.org, Dulshani Gunawardhana , stable@vger.kernel.org Subject: Re: [PATCH] btrfs: fix reversed warning condition in btrfs_delayed_inode_reserve_metadata Message-ID: <20140404134044.GU29256@twin.jikos.cz> Reply-To: dsterba@suse.cz References: <1396458780-10923-1-git-send-email-dsterba@suse.cz> <20140403053421.GB11484@localhost.localdomain> <20140403161840.GL29256@suse.cz> <20140404030315.GA24064@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20140404030315.GA24064@localhost.localdomain> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Fri, Apr 04, 2014 at 11:03:16AM +0800, Liu Bo wrote: > On Thu, Apr 03, 2014 at 06:18:40PM +0200, David Sterba wrote: > > On Thu, Apr 03, 2014 at 01:34:23PM +0800, Liu Bo wrote: > > > On Wed, Apr 02, 2014 at 07:13:00PM +0200, David Sterba wrote: > > > > Commit fae7f21cece9a4c181 ("btrfs: Use WARN_ON()'s return value in place of > > > > WARN_ON(1)") cleaned up WARN_ON usage and in one place reversed the condition > > > > that led to loads of warnings that were not supposed to occur. > > > > > > > > WARN_ON will trigger because it sees 'ret' though in the previous code > > > > did not reach the WARN_ON below. The correct pattern is > > > > > > > > - if (condition) > > > > + if (WARN_ON(condition)) > > > > > > > > CC: Dulshani Gunawardhana > > > > CC: # 3.13 > > > > Reported-by: Liu Bo > > > > Signed-off-by: David Sterba > > > > --- > > > > fs/btrfs/delayed-inode.c | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c > > > > index 451b00c86f6c..098af20abd88 100644 > > > > --- a/fs/btrfs/delayed-inode.c > > > > +++ b/fs/btrfs/delayed-inode.c > > > > @@ -649,7 +649,7 @@ static int btrfs_delayed_inode_reserve_metadata( > > > > goto out; > > > > > > > > ret = btrfs_block_rsv_migrate(src_rsv, dst_rsv, num_bytes); > > > > - if (!WARN_ON(ret)) > > > > + if (WARN_ON(!ret)) > > > > goto out; > > > > > > Oh sorry, I'd have to get my Reviewed-by back and give a NACK instead. > > > > > > With this patch, (ret = 0) triggers the WARNING, which is not right. > > > > Thanks for catching this, you're right, my patch was wrong. I must say > > the patch (fae7f21ce) made the code harder to read at some places, I > > don't see much help in removing plain WARN_ON(1) at this cost. > > I agree, I prefer the original code which is easier to understand, > > if (!ret) > goto out; > WARN_ON(1); > > > > > Back to the warning flood you observed, the comment under the warning > > says: > > > > 655 /* > > 656 * Ok this is a problem, let's just steal from the global rsv > > 657 * since this really shouldn't happen that often. > > 658 */ > > 659 ret = btrfs_block_rsv_migrate(&root->fs_info->global_block_rsv, > > 660 dst_rsv, num_bytes); > > > > so the question is why it does happen so often. > > > > A WARN_ON_ONCE hides the severity of the problem, so I'd rather suggest > > to put it under enospc_debug option so we can debug it and it does not > > bother users. As this is closer to the way you were going to fix that, > > I'm not sending a patch, take this as a review comment. > > The comment was based on some assumptions which could be wrong according to > my observation. Then the question is if the WARN_ON points to a problem or not.