linux-kernel - Re: [RFC PATCH v1 06/25] printk-rb: add blocking reader support

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Date:   Mon, 18 Feb 2019 15:05:38 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 06/25] printk-rb: add blocking reader support

On Tue 2019-02-12 15:29:44, John Ogness wrote:
> Add a blocking read function for readers. An irq_work function is
> used to signal the wait queue so that write notification can
> be triggered from any context.

I would be more precise what exacly is problematic in which context.
Something like:

An irq_work function is used because wake_up() cannot be called safely
from NMI and scheduler context.

> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  include/linux/printk_ringbuffer.h | 20 ++++++++++++++++
>  lib/printk_ringbuffer.c           | 49 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 69 insertions(+)
> 
> diff --git a/include/linux/printk_ringbuffer.h b/include/linux/printk_ringbuffer.h
> index 5fdaf632c111..106f20ef8b4d 100644
> --- a/include/linux/printk_ringbuffer.h
> +++ b/include/linux/printk_ringbuffer.h
> @@ -2,8 +2,10 @@
>  #ifndef _LINUX_PRINTK_RINGBUFFER_H
>  #define _LINUX_PRINTK_RINGBUFFER_H
>  
> +#include <linux/irq_work.h>
>  #include <linux/atomic.h>
>  #include <linux/percpu.h>
> +#include <linux/wait.h>
>  
>  struct prb_cpulock {
>  	atomic_t owner;
> @@ -22,6 +24,10 @@ struct printk_ringbuffer {
>  
>  	struct prb_cpulock *cpulock;
>  	atomic_t ctx;
> +
> +	struct wait_queue_head *wq;
> +	atomic_long_t wq_counter;
> +	struct irq_work *wq_work;
>  };
>  
>  struct prb_entry {
> @@ -59,6 +65,15 @@ struct prb_iterator {
>  #define DECLARE_STATIC_PRINTKRB(name, szbits, cpulockptr)		\
>  static char _##name##_buffer[1 << (szbits)]				\
>  	__aligned(__alignof__(long));					\
> +static DECLARE_WAIT_QUEUE_HEAD(_##name##_wait);				\
> +static void _##name##_wake_work_func(struct irq_work *irq_work)		\
> +{									\
> +	wake_up_interruptible_all(&_##name##_wait);			\
> +}									\

All ring buffers might share the same generic function, something like:

void prb_wake_readers_work_func(struct irq_work *irq_work)
{
	struct printk_ringbuffer *rb;

	rb = container_of(irq_work, struct printk_ring_buffer, wq_work);
	wake_up_interruptible_all(rb->wq);			\
}


> +static struct irq_work _##name##_wake_work = {				\
> +	.func = _##name##_wake_work_func,				\
> +	.flags = IRQ_WORK_LAZY,						\
> +};									\
>  static struct printk_ringbuffer name = {				\
>  	.buffer = &_##name##_buffer[0],					\
>  	.size_bits = szbits,						\
> diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
> index 1d1e886a0966..c2ddf4cb9f92 100644
> --- a/lib/printk_ringbuffer.c
> +++ b/lib/printk_ringbuffer.c
> @@ -185,6 +188,12 @@ void prb_commit(struct prb_handle *h)
>  	}
>  
>  	prb_unlock(rb->cpulock, h->cpu);
> +
> +	if (changed) {
> +		atomic_long_inc(&rb->wq_counter);
> +		if (wq_has_sleeper(rb->wq))
> +			irq_work_queue(rb->wq_work);
> +	}
>  }
>  
>  /*
> @@ -437,3 +446,43 @@ int prb_iter_next(struct prb_iterator *iter, char *buf, int size, u64 *seq)
>  
>  	return 1;
>  }
> +
> +/*
> + * prb_iter_wait_next: Advance to the next record, blocking if none available.
> + * @iter: Iterator tracking the current position.
> + * @buf: A buffer to store the data of the next record. May be NULL.
> + * @size: The size of @buf. (Ignored if @buf is NULL.)
> + * @seq: The sequence number of the next record. May be NULL.
> + *
> + * If a next record is already available, this function works like
> + * prb_iter_next(). Otherwise block interruptible until a next record is
> + * available.
> + *
> + * When a next record is available, @iter is advanced and (if specified)
> + * the data and/or sequence number of that record are provided.
> + *
> + * This function might sleep.
> + *
> + * Returns 1 if @iter was advanced, -EINVAL if @iter is now invalid, or
> + * -ERESTARTSYS if interrupted by a signal.
> + */
> +int prb_iter_wait_next(struct prb_iterator *iter, char *buf, int size, u64 *seq)
> +{
> +	unsigned long last_seen;
> +	int ret;
> +
> +	for (;;) {
> +		last_seen = atomic_long_read(&iter->rb->wq_counter);
> +
> +		ret = prb_iter_next(iter, buf, size, seq);
> +		if (ret != 0)
> +			break;
> +
> +		ret = wait_event_interruptible(*iter->rb->wq,
> +			last_seen != atomic_long_read(&iter->rb->wq_counter));

Do we really need yet another counter here?

I think that rb->seq might do the same job. Or if there is problem
with atomicity then rb->head might work as well. Or do I miss
anything?

Best Regards,
Petr

> +		if (ret < 0)
> +			break;
> +	}
> +
> +	return ret;
> +}
> -- 
> 2.11.0
>