Re: [RFC][PATCH 0/6] Use printk_safe context for TTY and UART port locks

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
	Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dmitry Vyukov <dvyukov@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-serial <linux-serial@vger.kernel.org>,
	SergeySenozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC][PATCH 0/6] Use printk_safe context for TTY and UART port locks
Date: Thu, 28 Jun 2018 11:55:12 +0900	[thread overview]
Message-ID: <20180628025512.GF481@jagdpanzerIV> (raw)
In-Reply-To: <CA+55aFzC5=awDhzOy9u_2FNaQmRprUTxj_KV2xO+GnpNAFg8MQ@mail.gmail.com>

On (06/20/18 12:38), Linus Torvalds wrote:
> On Wed, Jun 20, 2018 at 11:50 AM Sergey Senozhatsky
> <sergey.senozhatsky.work@gmail.com> wrote:
> >
> > It's not UART on its own that immediately calls into printk(), that would
> > be trivial to fix, it's all those subsystems that serial console driver
> > can call into.
> 
> We already have the whole PRINTK_SAFE_CONTEXT_MASK model that only
> adds it to a secondary buffer if you get recursion.  Why isn't that
> triggering? That's the whole point of it.

Linus, Alan, Steven,
are you on board with the patch set?
What shall I do to improve it?

PRINTK_SAFE_CONTEXT_MASK is what we answer nowadays when someone says
"printk causes deadlocks". We really can't remove all printk-s that can
cause uart->...->printk->uart recursion, and the only other option is to
use spin_trylock on uart_port->lock in every driver and discard con->write()
if we see that we have re-entered uart. I'd rather use per-CPU printk_safe
buffer in this case.

	-ss