linux-kernel - Re: [PATCH printk-rework 08/14] printk: add syslog

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Date:   Mon, 22 Feb 2021 17:43:56 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk-rework 08/14] printk: add syslog_lock

On 2021-02-22, Petr Mladek <pmladek@...e.com> wrote:
>>>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>>>> index 20c21a25143d..401df370832b 100644
>>>> --- a/kernel/printk/printk.c
>>>> +++ b/kernel/printk/printk.c
>>>> +/* Return a consistent copy of @syslog_seq. */
>>>> +static u64 read_syslog_seq_irq(void)
>>>> +{
>>>> +	u64 seq;
>>>> +
>>>> +	raw_spin_lock_irq(&syslog_lock);
>>>> +	seq = syslog_seq;
>>>> +	raw_spin_unlock_irq(&syslog_lock);
>>>
>>> Is there any particular reason to disable interrupts here?
>>>
>>> It would make sense only when the lock could be taken in IRQ
>>> context. Then we would need to always disable interrupts when
>>> the lock is taken. And if it is taken in IRQ context, we would
>>> need to safe flags.
>
> Note that console_lock was a spinlock in 2.3.15.pre1. I see it defined
> in kernel/printk.c as:
>
> spinlock_t console_lock = SPIN_LOCK_UNLOCKED;
>
> But it is a sleeping semaphore these days. As a result,
> register_console(), as it is now, must not be called in an interrupt
> context.

OK. So I will change read_syslog_seq_irq() to not disable interrupts. As
you suggested, we can fix the rest when we remove the safe buffers.

John Ogness