linux-kernel - Re: [RFC][PATCHv2 3/4] serial: introduce uart

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Message-ID: <20181208031249.GA443@jagdpanzerIV>
Date:   Sat, 8 Dec 2018 12:12:49 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Cc:     linux-kernel@...r.kernel.org, Daniel Wang <wonderfly@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers

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