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Date: Fri, 4 Oct 2024 16:29:12 -0700
From: Namhyung Kim <namhyung@kernel.org>
To: Song Liu <song@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Eduard Zingerman <eddyz87@gmail.com>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@fomichev.me>,
	Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, bpf@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Kees Cook <kees@kernel.org>
Subject: Re: [PATCH v4 bpf-next 1/3] bpf: Add kmem_cache iterator
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References: <20241002180956.1781008-1-namhyung@kernel.org>
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In-Reply-To: <CAPhsuW7NFHbt0yPxh81gkRo8q_z_6JSrGGGLXtPMqvrbxk6b5w@mail.gmail.com>

On Fri, Oct 04, 2024 at 02:46:43PM -0700, Song Liu wrote:
> On Fri, Oct 4, 2024 at 2:37 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > Hi Song,
> >
> > On Fri, Oct 04, 2024 at 01:33:19PM -0700, Song Liu wrote:
> [...]
> > > > +
> > > > +static void *kmem_cache_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > > > +{
> > > > +       struct kmem_cache *s = v;
> > > > +       struct kmem_cache *next = NULL;
> > > > +       bool destroy = false;
> > > > +
> > > > +       ++*pos;
> > > > +
> > > > +       mutex_lock(&slab_mutex);
> > > > +
> > > > +       if (list_last_entry(&slab_caches, struct kmem_cache, list) != s) {
> > > > +               next = list_next_entry(s, list);
> > > > +               if (next->refcount > 0)
> > > > +                       next->refcount++;
> > >
> > > What if next->refcount <=0? Shall we find next of next?
> >
> > The slab_mutex should protect refcount == 0 case so it won't see that.
> > The negative refcount means it's a boot_cache and we shouldn't touch the
> > refcount.
> 
> I see. Thanks for the explanation!
> 
> Please add a comment here, and maybe also add
> 
>   WARN_ON_ONCE(next ->refcount == 0).

Sure, thanks for your review!
Namhyung