linux-kernel - Re: Converting dev->mutex into dev->spinlock ?

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Date:   Sat, 4 Feb 2023 11:27:53 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>
Subject: Re: Converting dev->mutex into dev->spinlock ?

On Sun, Feb 05, 2023 at 01:12:12AM +0900, Tetsuo Handa wrote:
> On 2023/02/05 0:34, Alan Stern wrote:
> >> A few of examples:
> >>
> >>   https://syzkaller.appspot.com/bug?extid=2d6ac90723742279e101
> > 
> > It's hard to figure out what's wrong from looking at the syzbot report.  
> > What makes you think it is connected with dev->mutex?
> > 
> > At first glance, it seems that the ath6kl driver is trying to flush a 
> > workqueue while holding a lock or mutex that is needed by one of the 
> > jobs in the workqueue.  That's obviously never going to work, no matter 
> > what sort of lockdep validation gets used.
> 
> That lock is exactly dev->mutex where lockdep validation is disabled.
> If lockdep validation on dev->mutex were not disabled, we can catch
> possibility of deadlock before khungtaskd reports real deadlock as hung.
> 
> Lockdep validation on dev->mutex being disabled is really annoying, and
> I want to make lockdep validation on dev->mutex enabled; that is the
> "drivers/core: Remove lockdep_set_novalidate_class() usage" patch.

> Even if it is always safe to acquire a child device's lock while holding
> the parent's lock, disabling lockdep checks completely on device's lock is
> not safe.

I understand the problem you want to solve, and I understand that it
can be frustrating.  However, I do not believe you will be able to
solve this problem.

Alan Stern