From: Qian Cai <cai@lca.pw>
To: Dave Chinner <david@fromorbit.com>
Cc: darrick.wong@oracle.com, dchinner@redhat.com,
peterz@infradead.org, bfoster@redhat.com, hch@lst.de,
linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] xfs: silence lockdep false positives when freezing
Date: Wed, 09 Jan 2019 16:13:13 -0500 [thread overview]
Message-ID: <1547068393.6911.3.camel@lca.pw> (raw)
In-Reply-To: <20190109210111.GZ4205@dastard>
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.
prev parent reply other threads:[~2019-01-09 21:13 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20190106225639.GU4205@dastard>
2019-01-09 20:53 ` [PATCH] xfs: silence lockdep false positives when freezing Qian Cai
2019-01-09 21:01 ` Dave Chinner
2019-01-09 21:13 ` Qian Cai [this message]
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