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

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Date:   Thu, 28 Jun 2018 11:55:12 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        SergeySenozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 0/6] Use printk_safe context for TTY and UART port
 locks

On (06/20/18 12:38), Linus Torvalds wrote:
> On Wed, Jun 20, 2018 at 11:50 AM Sergey Senozhatsky
> <sergey.senozhatsky.work@...il.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