Re: PANIC: "Oops: 0000 [#1] PREEMPT SMP PTI" starting from 5.17 on dual socket Intel Xeon Gold servers

From: Tejun Heo <tj@kernel.org>
To: Jirka Hladky <jhladky@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	regressions@lists.linux.dev,
	Thorsten Leemhuis <regressions@leemhuis.info>,
	Justin Forbes <jforbes@fedoraproject.org>
Subject: Re: PANIC: "Oops: 0000 [#1] PREEMPT SMP PTI" starting from 5.17 on dual socket Intel Xeon Gold servers
Date: Thu, 21 Apr 2022 06:47:41 -1000	[thread overview]
Message-ID: <YmGKrd1BR9HSEy6q@slm.duckdns.org> (raw)
In-Reply-To: <CAE4VaGAQZcQzN8D+iwcBnP5vY=Ctmbh+oTikvONHir6JjTgpsw@mail.gmail.com>

Sorry about late reply.

On Wed, Apr 20, 2022 at 10:02:20AM +0200, Jirka Hladky wrote:
> > Based on your report, kernel was crashed due to kn_mondata was NULL
> >
> >   rdt_kill_sb
> >     rmdir_all_sub
> >       ..
> >       kernfs_remove(kn_mondata);
> >         struct kernfs_root *root = kernfs_root(kn); <-- crashed
> >
> >
> > Before the my patch[1], it worked like this.
> >
> >   rdt_kill_sb
> >     rmdir_all_sub
> >       ..
> >       kernfs_remove(kn_mondata);
> >         down_write(&kernfs_rwsem);
> >           if (!kn)
> >             return;
> >         up_write(&kernfs_rwsem);
> >
> > IOW, before, kernfs_remove worked with NULL argument via just bailing
> > but with the my patch[1], it doesn't work any longer.
> >
> > It makes me have questions for kernfs maintainers:
> >
> > Should kernfs_remove API support NULL parameter? If so, can we support
> > it atomically without old global kernfs_rwsem?
> >
> > [1] 393c3714081a, kernfs: switch global kernfs_rwsem lock to per-fs lock

Yes, I mean, kernfs_remove() used to support NULL arg, so it should do the
same after the locking change too. Can you send a patch?

Thanks.

-- 
tejun