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