linux-kernel - Re: [RT v5.11-rt7] WARNING at include/linux/seqlock.h:271 nft_counter

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Message-ID: <YDUPEsdMAMWjmm8Z@lx-t490>
Date:   Tue, 23 Feb 2021 15:20:02 +0100
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RT v5.11-rt7] WARNING at include/linux/seqlock.h:271
 nft_counter_eval

On Tue, Feb 23, 2021 at 02:53:40PM +0100, Juri Lelli wrote:
> On 23/02/21 12:00, Sebastian Andrzej Siewior wrote:
> > On 2021-02-23 11:49:07 [+0100], Juri Lelli wrote:
> > > Hi,
> > Hi,
> >
> > > I'm seeing the following splat right after boot (or during late boot
> > > phases) with v5.11-rt7 (LOCKDEP enabled).
> > …
> > > [   85.273588] WARNING: CPU: 5 PID: 1416 at include/linux/seqlock.h:271 nft_counter_eval+0x95/0x130 [nft_counter]
> > …
> > > [   85.273713] RIP: 0010:nft_counter_eval+0x95/0x130 [nft_counter]
> >
> > This is a per-CPU seqcount_t in net/netfilter/nft_counter.c which is
> > only protected by local_bh_disabled(). The warning expects preemption
> > to be disabled which is the case on !RT but not on RT.
> >
> > Not sure what to do about this. It is doing anything wrong as of now. It
> > is noisy.
>
> So, I'm a bit confused and I'm very likely missing details (still
> digesting the seqprop_ magic), but write_seqcount_being() has
>
>  if (seqprop_preemptible(s))
>      preempt_disable();
>
> which in this case (no lock associated) is defined to return false,

Preemption is disabled if and only if:

  1. It's a CONFIG_PREEMPT_RT=n system
  2. There's a lock associated with the sequence counter
  3. That lock is also preemptible (e.g., a mutex)

In your case, the 3 condititions are OFF. You're on a PREEMPT_RT=y
kernel and the sequence counter in question has no lock associated.

As Sebastian summarized, the error is just "noisy" at this point.

We will of course need to find a (mainline-friendly) way to let the
lockdep splat go away for -rt kernels. But for now, it's not harmful.

Good luck,

--
Ahmed S. Darwish