Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")

[Peter Zijlstra]

On Mon, Oct 26, 2020 at 11:26:49AM +0000, Filipe Manana wrote:
> Hello,
> 
> I've recently started to hit a warning followed by tasks hanging after
> attempts to freeze a filesystem. A git bisection pointed to the
> following commit:
> 
> commit 4d004099a668c41522242aa146a38cc4eb59cb1e
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date:   Fri Oct 2 11:04:21 2020 +0200
> 
>     lockdep: Fix lockdep recursion
> 
> This happens very reliably when running all xfstests with lockdep
> enabled, and the tested filesystem is btrfs (haven't tried other
> filesystems, but it shouldn't matter). The warning and task hangs always
> happen at either test generic/068 or test generic/390, and (oddly)
> always have to run all tests for it to trigger, running those tests
> individually on an infinite loop doesn't seem to trigger it (at least
> for a couple hours).
> 
> The warning triggered is at fs/super.c:__sb_start_write() which always
> results later in several tasks hanging on a percpu rw_sem:
> 
> https://pastebin.com/qnLvf94E
> 
> What happens is percpu_rwsem_is_held() is apparently returning a false
> positive,

That smells like the same issue reported here:

  https://lkml.kernel.org/r/20201022111700.GZ2651@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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?