On Wed, Oct 06, 2021 at 01:58:52PM +0200, 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@xxxxxxxxxxxxxxx # 4.0
> Reported-by: Fabio Estevam <festevam@xxxxxxx>
> Tested-by: Fabio Estevam <festevam@xxxxxxx>
> Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
Applied to wq/for-5.15-fixes.
Thanks.
--
tejun
