linux-kernel - Re: [PATCH] xfs: silence lockdep false positives when freezing

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Message-ID: <1547068393.6911.3.camel@lca.pw>
Date:   Wed, 09 Jan 2019 16:13:13 -0500
From:   Qian Cai <cai@....pw>
To:     Dave Chinner <david@...morbit.com>
Cc:     darrick.wong@...cle.com, dchinner@...hat.com, peterz@...radead.org,
        bfoster@...hat.com, hch@....de, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: silence lockdep false positives when freezing

On Thu, 2019-01-10 at 08:01 +1100, Dave Chinner wrote:
> On Wed, Jan 09, 2019 at 03:53:29PM -0500, Qian Cai wrote:
> > Easy to reproduce:
> > 
> > 1. run LTP oom02 workload to let kswapd acquire this locking order:
> >    fs_reclaim -> sb_internal.
> > 
> >  # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal
> > [00000000826b9172] &type->s_umount_key#27
> > [000000005fa8b2ac] sb_pagefaults
> > [0000000033f1247e] sb_internal
> > [000000009e9a9664] fs_reclaim
> > 
> > 2. freeze XFS.
> >   # fsfreeze -f /home
> > 
> > Dave mentioned that this is due to a lockdep limitation - "IOWs, this is
> > a false positive, caused by the fact that xfs_trans_alloc() is called
> > from both above and below memory reclaim as well as within /every level/
> > of freeze processing. Lockdep is unable to describe the staged flush
> > logic in the freeze process that prevents deadlocks from occurring, and
> > hence we will pretty much always see false positives in the freeze
> > path....". Hence, just temporarily disable lockdep in that path.
> 
> NACK. Turning off lockdep is not a solution, it just prevents
> lockdep from finding and reporting real issues.
> 

Well, it is a trade-off. It is turned on right after that path. All those false
positives leave unfixed are also going to render lockdep less useful.