From mboxrd@z Thu Jan  1 00:00:00 1970
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers
Date: Wed, 12 Dec 2018 12:08:38 +0100
Message-ID: <20181212110838.GA11361@kroah.com>
References: <20181016050428.17966-1-sergey.senozhatsky@gmail.com>
 <20181016050428.17966-4-sergey.senozhatsky@gmail.com>
 <20181208031249.GA443@jagdpanzerIV>
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To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>, Steven Rostedt <rostedt@goodmis.org>, Jiri Slaby <jslaby@suse.com>, linux-kernel@vger.kernel.org, Daniel Wang <wonderfly@google.com>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>, Peter Feiner <pfeiner@google.com>, linux-serial@vger.kernel.org, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
List-Id: linux-serial@vger.kernel.org

On Sat, Dec 08, 2018 at 12:12:49PM +0900, Sergey Senozhatsky wrote:
> 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?

If done in a sane manner, no objection from me.