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

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Message-ID: <20190109210111.GZ4205@dastard>
Date:   Thu, 10 Jan 2019 08:01:12 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Qian Cai <cai@....pw>
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 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.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com