linux-kernel - Re: [PATCH printk v4 06/27] printk: nbcon: Add callbacks to synchronize with driver

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Date: Thu, 18 Apr 2024 14:16:16 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, Steven Rostedt
 <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>,
 linux-kernel@...r.kernel.org, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v4 06/27] printk: nbcon: Add callbacks to
 synchronize with driver

On 2024-04-18, Petr Mladek <pmladek@...e.com> wrote:
> I am not sure how it is done in other parts of kernel code where
> RT needed to introduce some tricks. But I think that we should
> really start mentioning RT behavior in the commit messages and
> and comments where the RT mode makes huge changes.

Yes, our motivation is RT. But these semantics are not RT-specific. They
apply to the general kernel locking model. For example, even for a !RT
system, it is semantically incorrect to take a spin_lock while holding a
raw_spin_lock.

In the full PREEMPT_RT series I have tried to be careful about only
mentioning PREEMPT_RT when it is really PREEMPT_RT-specific. For example
[0][1][2].

[0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=1564af55a92c32fe215af35cf55cb9359c5fff30

[1] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=033b416ad25b17dc60d5f71c1a0b33a5fbc17639

[2] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=7929ba9e5c110148a1fcd8bd93d6a4eff37aa265

> The race could NOT happen in:
>
>    + NBCON_PRIO_PANIC context because it does not schedule

Yes.

>    + NBCON_PRIO_EMERGENCY context because we explicitly disable
>      preemption there

Yes.

>    + NBCON_NORMAL_PRIO context when we ALWAYS do nbcon_try_acquire()
>      under con->device() lock. Here the con->device_lock() serializes
>      nbcon_try_acquire() calls even between running tasks.

The nbcon_legacy_emit_next_record() printing as NBCON_NORMAL_PRIO is a
special situation where write_atomic() is used. It is safe because it
disables hard interrupts and is never called from NMI context.

nbcon_atomic_flush_pending() as NBCON_NORMAL_PRIO is safe in !NMI
because it also disables hard interrupts. However,
nbcon_atomic_flush_pending() could be called in NMI with
NBCON_NORMAL_PRIO. I need to think about this case.

John