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Date:   Mon, 26 Oct 2020 13:55:24 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Filipe Manana <fdmanana@suse.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Jan Kara <jack@suse.cz>,
        David Sterba <dsterba@suse.com>
Subject: Re: possible lockdep regression introduced by 4d004099a668
 ("lockdep: Fix lockdep recursion")
Message-ID: <20201026125524.GP2594@hirez.programming.kicks-ass.net>
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 <20201026114009.GN2594@hirez.programming.kicks-ass.net>
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On Mon, Oct 26, 2020 at 11:56:03AM +0000, Filipe Manana wrote:
> > That smells like the same issue reported here:
> > 
> >   https://lkml.kernel.org/r/20201022111700.GZ2651@hirez.programming.kicks-ass.net
> > 
> > Make sure you have commit:
> > 
> >   f8e48a3dca06 ("lockdep: Fix preemption WARN for spurious IRQ-enable")
> > 
> > (in Linus' tree by now) and do you have CONFIG_DEBUG_PREEMPT enabled?
> 
> Yes, CONFIG_DEBUG_PREEMPT is enabled.

Bummer :/

> I'll try with that commit and let you know, however it's gonna take a
> few hours to build a kernel and run all fstests (on that test box it
> takes over 3 hours) to confirm that fixes the issue.

*ouch*, 3 hours is painful. How long to make it sick with the current
kernel? quicker I would hope?

> Thanks for the quick reply!

Anyway, I don't think that commit can actually explain the issue :/

The false positive on lockdep_assert_held() happens when the recursion
count is !0, however we _should_ be having IRQs disabled when
lockdep_recursion > 0, so that should never be observable.

My hope was that DEBUG_PREEMPT would trigger on one of the
__this_cpu_{inc,dec}(lockdep_recursion) instance, because that would
then be a clear violation.

And you're seeing this on x86, right?

Let me puzzle moar..