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Date:   Wed, 7 Jul 2021 11:34:17 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, skhan@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+e6d5398a02c516ce5e70@syzkaller.appspotmail.com
Subject: Re: [PATCH v2 1/2] fcntl: fix potential deadlocks for
 &fown_struct.lock
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On Wed, Jul 07, 2021 at 05:31:06PM +0200, Greg KH wrote:
> On Wed, Jul 07, 2021 at 11:19:36AM -0400, J. Bruce Fields wrote:
> > On Wed, Jul 07, 2021 at 05:06:45PM +0200, Greg KH wrote:
> > > On Wed, Jul 07, 2021 at 09:51:29AM -0400, J. Bruce Fields wrote:
> > > > On Wed, Jul 07, 2021 at 07:40:47AM -0400, Jeff Layton wrote:
> > > > > On Wed, 2021-07-07 at 12:51 +0200, Greg KH wrote:
> > > > > > On Wed, Jul 07, 2021 at 06:44:42AM -0400, Jeff Layton wrote:
> > > > > > > On Wed, 2021-07-07 at 08:05 +0200, Greg KH wrote:
> > > > > > > > On Wed, Jul 07, 2021 at 10:35:47AM +0800, Desmond Cheong Zhi Xi wrote:
> > > > > > > > > +	WARN_ON_ONCE(irqs_disabled());
> > > > > > > > 
> > > > > > > > If this triggers, you just rebooted the box :(
> > > > > > > > 
> > > > > > > > Please never do this, either properly handle the problem and return an
> > > > > > > > error, or do not check for this.  It is not any type of "fix" at all,
> > > > > > > > and at most, a debugging aid while you work on the root problem.
> > > > > > > > 
> > > > > > > > thanks,
> > > > > > > > 
> > > > > > > > greg k-h
> > > > > > > 
> > > > > > > Wait, what? Why would testing for irqs being disabled and throwing a
> > > > > > > WARN_ON in that case crash the box?
> > > > > > 
> > > > > > If panic-on-warn is enabled, which is a common setting for systems these
> > > > > > days.
> > > > > 
> > > > > Ok, that makes some sense.
> > > > 
> > > > Wait, I don't get it.
> > > > 
> > > > How are we supposed to decide when to use WARN, when to use BUG, and
> > > > when to panic?  Do we really want to treat them all as equivalent?  And
> > > > who exactly is turning on panic-on-warn?
> > > 
> > > You never use WARN or BUG, unless the system is so messed up that you
> > > can not possibly recover from the issue.
> > 
> > I've heard similar advice for BUG before, but this is the first I've
> > heard it for WARN.  Do we have any guidelines for how to choose between
> > WARN and BUG?
> 
> Never use either :)

I can't tell if you're kidding.  Is there some plan to remove them?

There are definitely cases where I've been able to resolve a problem
more quickly because I got a backtrace from a WARN.

--b.