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From: Richard Purdie <>
To: Mark Brown <>, Tejun Heo <>
Cc: Paul Gortmaker <>,,,
	Al Viro <>,
	Zefan Li <>,
	Johannes Weiner <>,
Subject: Re: [PATCH] cgroup1: fix leaked context root causing sporadic NULL deref in LTP
Date: Wed, 30 Jun 2021 23:31:06 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, 2021-06-30 at 17:10 +0100, Mark Brown wrote:
> On Wed, Jun 16, 2021 at 11:23:34AM -0400, Tejun Heo wrote:
> > On Wed, Jun 16, 2021 at 08:51:57AM -0400, Paul Gortmaker wrote:
> > > A fix would be to not leave the stale reference in fc->root as follows:
> > >    --------------
> > >                   dput(fc->root);
> > >   +               fc->root = NULL;
> > >                   deactivate_locked_super(sb);
> > >    --------------
> > > ...but then we are just open-coding a duplicate of fc_drop_locked() so we
> > > simply use that instead.
> > As this is unlikely to be a real-world problem both in probability and
> > circumstances, I'm applying this to cgroup/for-5.14 instead of
> > cgroup/for-5.13-fixes.
> FWIW at Arm we've started seeing what appears to be this issue blow up
> very frequently in some of our internal LTP CI runs against -next, seems
> to be mostly on lower end platforms.  We seem to have started finding it
> at roughly the same time that the Yocto people did, I guess some other
> change made it more likely to trigger.  Not exactly real world usage
> obviously but it's creating quite a bit of noise in testing which is
> disruptive so it'd be good to get it into -next as a fix.

It is a horrible bug to debug as you end up with "random" failures on the 
systems which are hard to pin down. Along with the RCU stall hangs it
was all a bit of a nightmare.

Out of interest are you also seeing the proc01 test hang on a non-blocking
read of /proc/kmsg periodically?

I've not figured out a way to reproduce it at will yet and it seems strace
was enough to unblock it. It seems arm specific.



  reply	other threads:[~2021-06-30 22:31 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-16 12:51 [PATCH] cgroup1: fix leaked context root causing sporadic NULL deref in LTP Paul Gortmaker
2021-06-16 15:23 ` Tejun Heo
2021-06-30 16:10   ` Mark Brown
2021-06-30 22:31     ` Richard Purdie [this message]
2021-07-01 12:11       ` Mark Brown
2021-07-01 12:43         ` Richard Purdie
2021-07-12 17:34 ` Tejun Heo

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