linux-kernel - Re: [PATCH v3] serial: imx: Suppress false positive sysrq lockdep warning

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Date:   Wed, 06 Oct 2021 07:52:25 -0300
From:   Fabio Estevam <festevam@...x.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     gregkh@...uxfoundation.org, michael@...le.cc,
        linux-serial@...r.kernel.org, marex@...x.de,
        u.kleine-koenig@...gutronix.de, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3]  serial: imx: Suppress false positive sysrq lockdep
 warning

Hi Johan,

On 06/10/2021 05:10, Johan Hovold wrote:

> Ok, thanks for testing. The above is what I meant and it does fix the
> false-positive lockdep splat which motivated
> uart_unlock_and_check_sysrq() to be added in the first place.
> 
> Looking closer at the splat you reported (which you've edited quite
> heavily), it becomes apparent that you are now hitting a different
> locking issue. And it's not a false positive this time.
> 
> There a problem with the workqueue debugging code, which unless fixed 
> at
> the source, would prevent any console driver from queueing work while
> holding a lock also taken in their write paths. And
> tty_flip_buffer_push() is just one example of many.
> 
> I can easily reproduce the splat with another serial driver, and I've
> also been able to trigger the actual deadlock.
> 
> I've prepared a patch that takes care of the workqueue state dumping,
> which I'll send as a reply to this mail. Would you mind giving it a 
> spin
> with the imx driver as well?

Yes, after applying only your patch I no longer get the lockdep
splat. I have replied with my Tested-by, thanks.