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

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Date:   Wed, 20 Jun 2018 13:28:47 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>,
        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.

This is exactly what I'm doing in my patch set.
PRINTK_SAFE_CONTEXT_MASK so far worked *one* way only: when we start
from printk.c

IOW:

      printk -> printk_safe_mask -> vsprinf -> printk

But we also can have printk-related deadlocks the *other* way
around. For instance:

      uart -> printk -> uart

printk_safe_mask is not triggering there because we don't use
printk_safe in uart / tty yet. And this is what I do in my
patch set - extend printk_safe usage.

The patch set does not add any _new_ locks or locking rules.
It just replaces the existing

		spin_lock(a)
with
		prinkt_safe_enter();
		spin_lock(a)

and
		spin_unlock(a)
with
		spin_unlock(a)
		printk_safe_exit();

and that's it.

So now we use printk_safe mechanism to avoid another bunch of
deadlock scenarious: which don't start from printk, but from
parts of the kernel which printk eventually calls.

	-ss