Re: [RFC PATCH v1 1/3] locking/lockdep: Fix false warning of check_wait_context()

[Xiongwei Song <sxwjean@gmail.com>]

On Mon, Jul 12, 2021 at 9:04 PM Waiman Long <llong@redhat.com> wrote:
>
> On 7/12/21 4:18 AM, Xiongwei Song wrote:
> > On Mon, Jul 12, 2021 at 12:43 AM Waiman Long <llong@redhat.com> wrote:
> >> On 7/11/21 10:14 AM, Xiongwei Song wrote:
> >>> From: Xiongwei Song <sxwjean@gmail.com>
> >>>
> >>> We now always get a "Invalid wait context" warning with
> >>> CONFIG_PROVE_RAW_LOCK_NESTING=y, see the full warning below:
> >>>
> >>>        [    0.705900] =============================
> >>>        [    0.706002] [ BUG: Invalid wait context ]
> >>>        [    0.706180] 5.13.0+ #4 Not tainted
> >>>        [    0.706349] -----------------------------
> >> I believe the purpose of CONFIG_PROVE_RAW_LOCK_NESTING is experimental
> >> and it is turned off by default. Turning it on can cause problem as
> >> shown in your lockdep splat. Limiting it to just PREEMPT_RT will defeat
> >> its purpose to find potential spinlock nesting problem in non-PREEMPT_RT
> >> kernel.
> > As far as I know, a spinlock can nest another spinlock. In
> > non-PREEMPT_RT kernel
> > spin_lock and raw_spin_lock are same , so here acquiring a spin_lock in hardirq
> > context is acceptable, the warning is not needed. My knowledge on this
> > is not enough,
> > Will dig into this.
> >
> >> The point is to fix the issue found,
> > Agree. I thought there was a spinlock usage issue, but by checking
> > deactivate_slab context,
> > looks like the spinlock usage is well. Maybe I'm missing something?
>
> Yes, spinlock and raw spinlock are the same in non-RT kernel. They are
> only different in RT kernel. However, non-RT kernel is also more heavily
> tested than the RT kernel counterpart. The purpose of this config option
> is to expose spinlock nesting problem in more areas of the code. If you
> look at the config help text of PROVE_RAW_LOCK_NESTING:
>
>          help
>           Enable the raw_spinlock vs. spinlock nesting checks which ensure
>           that the lock nesting rules for PREEMPT_RT enabled kernels are
>           not violated.
>
>           NOTE: There are known nesting problems. So if you enable this
>           option expect lockdep splats until these problems have been fully
>           addressed which is work in progress. This config switch allows to
>           identify and analyze these problems. It will be removed and the
>           check permanentely enabled once the main issues have been fixed.
>
>           If unsure, select N.
Yes, I checked before sending patch, but didn't understand everything.
Thanks, :-).

> So lockdep splat is expected. It will take time to address all the
> issues found.
Ok.

Regards,
Xiongwei
>
> Cheers,
> Longman
>