Re: [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers

Sergey Senozhatsky
2018-12-08T03:12:49Z

On (10/16/18 14:04), Sergey Senozhatsky wrote:
[..]
> - The first entry point is console ->write() callback, which we call
> from printk(). A possible deadlock scenario there is:
>
> CPU0
> <NMI>
> spin_lock_irqsave(&port->lock, flags) << deadlock
> serial_foo_write()
> call_console_drivers()
> console_unlock()
> console_flush_on_panic()
> panic()
> <NMI/>
> spin_lock_irqsave(&port->lock, flags)
> serial_foo_write()
> call_console_drivers()
> console_unlock()
> printk()
> ...

[..]
> - The rest (of entry points) requires a bit different handling.
> Let's take a look at the following backtrace:
>
> CPU0
> <IRQ>
> spin_lock_irqsave(&port->lock, flags) << deadlock
> serial_foo_write()
> call_console_drivers()
> console_unlock()
> printk()
> __queue_work()
> tty_flip_buffer_push()
> spin_lock_irqsave(&port->lock, flags)
> serial_foo_handle_IRQ()
> <IRQ/>
>
> Serial drivers invoke tons of core kernel functions - WQ, MM, etc. All
> of which may printk() in various cases. So we can't really just
> "remove those printk-s". The simples way to address this seems to be
> PRINTK_SAFE_CONTEXT_MASK.

serial/UART and printk guys, sorry to bother you, do you hate this
idea of removing console_driver->CORE KERNEL->printk->console_driver
deadlock path? Or is there any chance we can move forward?

-ss