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

Petr Mladek
2021-08-04T09:52:53Z

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@linutronix.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