linux-kernel - Re: [PATCH printk v1 02/10] printk: rename printk cpulock API and always disable interrupts

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Date:   Wed, 4 Aug 2021 11:52:49 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v1 02/10] printk: rename printk cpulock API and
 always disable interrupts

On Tue 2021-08-03 15:18:53, John Ogness wrote:
> The printk cpulock functions use local_irq_disable(). This means that
> hardware interrupts are also disabled on PREEMPT_RT. To make this
> clear, rename the functions to use the raw_ prefix:
> 
>     raw_printk_cpu_lock_irqsave(flags);
>     raw_printk_cpu_unlock_irqrestore(flags);
> 
> Also, these functions were a NOP for !CONFIG_SMP. But for !CONFIG_SMP
> they still need to disable hardware interrupts. So modify them
> appropriately for this.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  include/linux/printk.h | 30 ++++++++++++++----------------
>  lib/dump_stack.c       |  4 ++--
>  lib/nmi_backtrace.c    |  4 ++--
>  3 files changed, 18 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 259af4f97f50..ac738d1d9934 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -280,17 +280,22 @@ static inline void dump_stack(void)
>  extern int __printk_cpu_trylock(void);
>  extern void __printk_wait_on_cpu_lock(void);
>  extern void __printk_cpu_unlock(void);
> +#else
> +#define __printk_cpu_trylock()		1
> +#define __printk_wait_on_cpu_lock()
> +#define __printk_cpu_unlock()
> +#endif /* CONFIG_SMP */

IMHO, it is not obvious that

		while (!__printk_cpu_trylock())		\
			__printk_wait_on_cpu_lock();	\

does nothing in the !CONFIG_SMP case. Please, make it more obvious.
I suggest to define:

#ifdef CONFIG_SMP

#define __printk_cpu_lock()				\
	do {						\
		while (!__printk_cpu_trylock())		\
			__printk_wait_on_cpu_lock();	\
	} while (0)

#else

#define __printk_cpu_lock()

#endif

/**
 * raw_printk_cpu_lock_irqsave() - Disable interrupts and acquire the printk
 *                                 cpu-reentrant spinning lock.
 * @flags: Stack-allocated storage for saving local interrupt state,
 *         to be passed to raw_printk_cpu_unlock_irqrestore().
 *
 * If the lock is owned by another CPU, spin until it becomes available.
 */
#define raw_printk_cpu_lock_irqsave(flags)	\
	do {					\
		local_irq_save(flags);		\
		__printk_cpu_lock();		\
	} while (0)


Best Regards,
Petr