linux-kernel - Re: kvm+nouveau induced lockdep gripe

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Date:   Mon, 26 Oct 2020 18:31:07 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Skeggs <bskeggs@...hat.com>
Subject: Re: kvm+nouveau induced lockdep  gripe

On 2020-10-24 13:00:00 [+0800], Hillf Danton wrote:
> 
> Hmm...curious how that word went into your mind. And when?
> > [   30.457363]
> >                other info that might help us debug this:
> > [   30.457369]  Possible unsafe locking scenario:
> > 
> > [   30.457375]        CPU0
> > [   30.457378]        ----
> > [   30.457381]   lock(&mgr->vm_lock);
> > [   30.457386]   <Interrupt>
> > [   30.457389]     lock(&mgr->vm_lock);
> > [   30.457394]
> >                 *** DEADLOCK ***
> > 
> > <snips 999 lockdep lines and zillion ATOMIC_SLEEP gripes>

The backtrace contained the "normal" vm_lock. What should follow is the
backtrace of the in-softirq usage.

> 
> Dunno if blocking softint is a right cure.
> 
> --- a/drivers/gpu/drm/drm_vma_manager.c
> +++ b/drivers/gpu/drm/drm_vma_manager.c
> @@ -229,6 +229,7 @@ EXPORT_SYMBOL(drm_vma_offset_add);
>  void drm_vma_offset_remove(struct drm_vma_offset_manager *mgr,
>  			   struct drm_vma_offset_node *node)
>  {
> +	local_bh_disable();

There is write_lock_bh(). However changing only one will produce the
same backtrace somewhere else unless all other users already run BH
disabled region.

>  	write_lock(&mgr->vm_lock);
>  
>  	if (drm_mm_node_allocated(&node->vm_node)) {

Sebastian