linux-kernel - Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

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Message-ID: <877e9bxf3l.fsf@linutronix.de>
Date:   Mon, 24 Jun 2019 15:55:26 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

Hi,

I would like to point out an important typo related to memory
barriers...

On 2019-06-07, John Ogness <john.ogness@...utronix.de> wrote:
> diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..d0b2b6a549b0
> --- /dev/null
> +++ b/lib/printk_ringbuffer.c
[...]
> +static struct prb_descr *remove_oldest_descr(struct printk_ringbuffer *rb)
> +{
> +	struct prb_list *l = &rb->descr_list;
> +	unsigned long oldest_id;
> +	struct prb_descr *d;
> +	unsigned long next;
> +
> +	for (;;) {
> +		oldest_id = READ_ONCE(l->oldest);
> +
> +		/* list empty */
> +		if (oldest_id == EOL)
> +			return NULL;
> +
> +		d = TO_DESCR(rb, oldest_id);
> +
> +		/* only descriptors with _invalid_ data can be removed */
> +		if (data_valid(rb, READ_ONCE(rb->data_list.oldest),
> +			       READ_ONCE(rb->data_list.newest),
> +			       READ_ONCE(d->data),
> +			       READ_ONCE(d->data_next))) {
> +			return NULL;
> +		}
> +
> +		/*
> +		 * MB6: synchronize link descr
> +		 *
> +		 * In particular: l->oldest is loaded as a data dependency so
> +		 * d->next and the following l->oldest will load afterwards,
> +		 * respectively.
> +		 */
> +		next = smp_load_acquire(&d->next);
> +
> +		if (next == EOL && READ_ONCE(l->oldest) == oldest_id) {
> +			/*
> +			 * The oldest has no next, so this is a list of one
> +			 * descriptor. Lists must always have at least one
> +			 * descriptor.
> +			 */
> +			return NULL;
> +		}
> +
> +		if (cmpxchg(&l->oldest, oldest_id, next) == oldest_id) {
> +			/* removed successfully */
> +			break;
> +		}

This is supposed to be cmpxchg_relaxed(), not cmpxchg(). I did not
intend to include the general mb() memory barriers around the RMW
operation. (For some reason I thought _relaxed was the default.) Sorry.

John Ogness