Re: [PATCH] workqueue: fix state-dump console deadlock

[Fabio Estevam <festevam@denx.de>]

Hi Johan,

On 06/10/2021 05:11, Johan Hovold wrote:
> Console drivers often queue work while holding locks also taken in 
> their
> console write paths, something which can lead to deadlocks on SMP when
> dumping workqueue state (e.g. sysrq-t or on suspend failures).
> 
> For serial console drivers this could look like:
> 
> 	CPU0				CPU1
> 	----				----
> 
> 	show_workqueue_state();
> 	  lock(&pool->lock);		<IRQ>
> 	  				  lock(&port->lock);
> 					  schedule_work();
> 					    lock(&pool->lock);
> 	  printk();
> 	    lock(console_owner);
> 	    lock(&port->lock);
> 
> where workqueues are, for example, used to push data to the line
> discipline, process break signals and handle modem-status changes. Line
> disciplines and serdev drivers can also queue work on write-wakeup
> notifications, etc.
> 
> Reworking every console driver to avoid queuing work while holding 
> locks
> also taken in their write paths would complicate drivers and is neither
> desirable or feasible.
> 
> Instead use the deferred-printk mechanism to avoid printing while
> holding pool locks when dumping workqueue state.
> 
> Note that there are a few WARN_ON() assertions in the workqueue code
> which could potentially also trigger a deadlock. Hopefully the ongoing
> printk rework will provide a general solution for this eventually.
> 
> This was originally reported after a lockdep splat when executing
> sysrq-t with the imx serial driver.
> 
> Fixes: 3494fc30846d ("workqueue: dump workqueues on sysrq-t")
> Cc: stable@vger.kernel.org	# 4.0
> Reported-by: Fabio Estevam <festevam@denx.de>
> Signed-off-by: Johan Hovold <johan@kernel.org>

With this patch applied, I no longer get the lockdep splat when 
executing
sysrq-t with the imx serial driver, thanks:

Tested-by: Fabio Estevam <festevam@denx.de>