linux-kernel - Re: [PATCH] printk: defer_console_output: do not assume interrupts disabled

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Date:   Thu, 3 Feb 2022 12:33:35 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: defer_console_output: do not assume interrupts
 disabled

On 2022-02-03 12:35:15 [+0106], John Ogness wrote:
> With commit 458df9fd4815 ("printk: remove separate printk_sched
> buffers and use printk buf instead") defer_console_output() was
> changed to no longer disable interrupts. However, it used the
> function __this_cpu_or(), which assumes interrupts disabled. That
> also should have been changed to this_cpu_or().
> 
> Although defer_console_output() is mostly used when interrupts are
> disabled, this is not always the case. One example is
> get_random_bytes(), which is often called with interrupts enabled.

It is okay for __this_cpu_or() to be invoked with disabled preemption if
that is true for all callers. Disabled interrupts or not is not
important as long as there are no callers in_irq(). I believe that there
are callers in_irq() and therefore this_cpu_or() is correct.

>   get_random_bytes()
>     warn_unseeded_randomness()
>       printk_deferred()
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Sebastian