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

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Date:   Tue, 19 Jun 2018 17:59:33 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 0/6] Use printk_safe context for TTY and UART port
 locks

On (06/19/18 10:30), Petr Mladek wrote:
> It is re-entrant via printk(). I mean that any printk() called inside
> the locked sections might cause recursion if the same lock is needed
> also by con->write() callbacks.

Perhaps Alan meant that we cannot return back to tty once we passed
the control from tty to printk -> uart serial console. So tty is
probably (but I didn't check) not re-entrant, but uart definitely is
re-entrant: IRQ -> uart console -> tty -> printk -> uart console.

The patch set attempts to address several issues, and re-entrant uart
is just one of them.

[..]
> This patchset forces safe context around TTY and UART locks.

Right.

> In fact, the deferred context would be enough to prevent
> all the mentioned deadlocks.

Agree.
But we can leave it as a printk_safe implementation detail and
change it later, outside of this patch, to avoid further confusion.

> IMHO, the only question is if people might get familiar with
> yet another spin_lock API.

Right. That's why I thought about renaming uart_port and tty_port
->lock to ->____lock. So the naming will suggest [hopefully] that
those locks are not meant to be used directly [anymore] and instead
people should use uart_port_lock/tty_port_lock API.

	-ss