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Date:   Thu, 3 Oct 2019 13:27:09 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Qian Cai <cai@lca.pw>
cc:     akpm@linux-foundation.org, cl@linux.com, penberg@kernel.org,
        tj@kernel.org, vdavydov.dev@gmail.com, hannes@cmpxchg.org,
        guro@fb.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/slub: fix a deadlock in show_slab_objects()
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On Thu, 3 Oct 2019, Qian Cai wrote:

> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 42c1b3af3c98..922cdcf5758a 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -4838,7 +4838,15 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
> > >  		}
> > >  	}
> > >  
> > > -	get_online_mems();
> > > +/*
> > > + * It is not possible to take "mem_hotplug_lock" here, as it has already held
> > > + * "kernfs_mutex" which could race with the lock order:
> > > + *
> > > + * mem_hotplug_lock->slab_mutex->kernfs_mutex
> > > + *
> > > + * In the worest case, it might be mis-calculated while doing NUMA node
> > > + * hotplug, but it shall be corrected by later reads of the same files.
> > > + */
> > >  #ifdef CONFIG_SLUB_DEBUG
> > >  	if (flags & SO_ALL) {
> > >  		struct kmem_cache_node *n;
> > 
> > No objection to removing the {get,put}_online_mems() but the comment 
> > doesn't match the kernel style.  I actually don't think we need the 
> > comment at all, actually.
> 
> I am a bit worry about later someone comes to add the lock back as he/she
> figures out that it could get more accurate statistics that way, but I agree it
> is probably an overkill.
> 

Maybe just a small comment that follows the kernel coding style?