Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c

[John Ogness <john.ogness@linutronix.de>]

On 2021-05-31, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 114e9963f903..98feead621ff 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3531,4 +3531,96 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
>  }
>  EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
>  
> +#ifdef CONFIG_SMP
> +static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
> +
> +/*
> + * printk_cpu_lock: Acquire the printk cpu-reentrant spinning lock.
> + * @cpu_store: A buffer to store lock state.
> + * @flags: A buffer to store irq state.
> + *
> + * If no processor has the lock, the calling processor takes the lock and
> + * becomes the owner. If the calling processor is already the owner of the
> + * lock, this function succeeds immediately. If the lock is locked by another
> + * processor, that function spins until the calling processor becomes the
> + * owner.
> + *
> + * It is safe to call this function from any context and state.
> + */
> +void printk_cpu_lock(unsigned int *cpu_store, unsigned long *flags)
> +{
> +	unsigned int cpu;
> +
> +	for (;;) {
> +		cpu = get_cpu();
> +
> +		*cpu_store = atomic_read(&printk_cpulock_owner);
> +
> +		if (*cpu_store == -1) {
> +			local_irq_save(*flags);
> +
> +			/*
> +			 * Guarantee loads an stores from the previous lock
> +			 * owner are visible to this CPU once it is the lock
> +			 * owner. This pairs with cpu_lock:A.

The above paragraph is totally broken. It should read:

  +			 * Guarantee loads and stores from the previous lock
  +			 * owner are visible to this CPU once it is the lock
  +			 * owner. This pairs with cpu_unlock:B.

John Ogness

> +			 *
> +			 * Memory barrier involvement:
> +			 *
> +			 * If cpu_lock:A reads from cpu_unlock:B, then
> +			 * cpu_lock:B reads from cpu_unlock:A.
> +			 *
> +			 * Relies on:
> +			 *
> +			 * RELEASE from cpu_unlock:A to cpu_unlock:B
> +			 *    matching
> +			 * ACQUIRE from cpu_lock:A to cpu_lock:B
> +			 */
> +			if (atomic_try_cmpxchg_acquire(&printk_cpulock_owner,
> +						       cpu_store, cpu)) { /* LMM(cpu_lock:A) */
> +
> +				/* This CPU begins loading/storing data: LMM(cpu_lock:B) */
> +				break;
> +			}
> +
> +			local_irq_restore(*flags);
> +
> +		} else if (*cpu_store == cpu) {
> +			break;
> +		}
> +
> +		put_cpu();
> +		cpu_relax();
> +	}
> +}
> +EXPORT_SYMBOL(printk_cpu_lock);
> +
> +/*
> + * printk_cpu_unlock: Release the printk cpu-reentrant spinning lock.
> + * @cpu_store: The current lock state.
> + * @flags: The current irq state.
> + *
> + * Release the lock. The calling processor must be the owner of the lock.
> + *
> + * It is safe to call this function from any context and state.
> + */
> +void printk_cpu_unlock(unsigned int cpu_store, unsigned long flags)
> +{
> +	if (cpu_store == -1) {
> +		/* This CPU is finished loading/storing data: LMM(cpu_unlock:A) */
> +
> +		/*
> +		 * Guarantee loads an stores from this CPU when it is the lock
> +		 * owner are visible to the next lock owner. This pairs with
> +		 * cpu_lock:A.
> +		 */
> +		atomic_set_release(&printk_cpulock_owner, cpu_store); /* LMM(cpu_unlock:B) */
> +
> +		local_irq_restore(flags);
> +	}
> +
> +	put_cpu();
> +}
> +EXPORT_SYMBOL(printk_cpu_unlock);
> +#endif /* CONFIG_SMP */
> +
>  #endif