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

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Date:   Tue, 18 Jun 2019 13:47:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.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

On Fri, Jun 07, 2019 at 06:29:48PM +0206, John Ogness wrote:
> +#define DATAARRAY_SIZE(rb) (1 << rb->data_array_size_bits)
> +#define DATAARRAY_SIZE_BITMASK(rb) (DATAARRAY_SIZE(rb) - 1)

*phew* no comments on those..

I think the kernel typically uses _MASK instead of _BITMASK for this
though.

> +/**
> + * DATA_INDEX() - Determine the data array index from logical position.
> + * @rb: The associated ringbuffer.
> + * @lpos: The logical position (data/data_next).
> + */
> +#define DATA_INDEX(rb, lpos) (lpos & DATAARRAY_SIZE_BITMASK(rb))
> +
> +/**
> + * DATA_WRAPS() - Determine how many times the data array has wrapped.
> + * @rb: The associated ringbuffer.
> + * @lpos: The logical position (data/data_next).
> + *
> + * The number of wraps is useful when determining if one logical position
> + * is overtaking the data array index another logical position.
> + */
> +#define DATA_WRAPS(rb, lpos) (lpos >> rb->data_array_size_bits)
> +
> +/**
> + * DATA_THIS_WRAP_START_LPOS() - Get the position at the start of the wrap.
> + * @rb: The associated ringbuffer.
> + * @lpos: The logical position (data/data_next).
> + *
> + * Given a logical position, return the logical position if backed up to the
> + * beginning (data array index 0) of the current wrap. This is used when a
> + * data block wraps and therefore needs to begin at the beginning of the data
> + * array (for the next wrap).
> + */
> +#define DATA_THIS_WRAP_START_LPOS(rb, lpos) \
> +	(DATA_WRAPS(rb, lpos) << rb->data_array_size_bits)

That's more easily written as: ((lpos) & ~MASK(rb))

> +
> +#define DATA_ALIGN sizeof(long)
> +#define DATA_ALIGN_SIZE(sz) \
> +	((sz + (DATA_ALIGN - 1)) & ~(DATA_ALIGN - 1))

We have ALIGN() for that

> +
> +#define DESCR_COUNT_BITMASK(rb) (rb->descr_max_count - 1)

I think the kernel typically uses 'DESC' as shorthand for Descriptor.
Idem on the MASK vs BITMASK thing.

> +
> +/**
> + * DESCR_INDEX() - Determine the descriptor array index from the id.
> + * @rb: The associated ringbuffer.
> + * @id: The descriptor id.
> + */
> +#define DESCR_INDEX(rb, id) (id & DESCR_COUNT_BITMASK(rb))
> +
> +#define TO_DATABLOCK(rb, lpos) \
> +	((struct prb_datablock *)&rb->data_array[DATA_INDEX(rb, lpos)])

If I were paranoid, I'd point out that this evaluates @rb twice, and
doesn't have the macro arguments in parens.

> +#define TO_DESCR(rb, id) \
> +	(&rb->descr_array[DESCR_INDEX(rb, id)])
> +
> +/**
> + * data_valid() - Check if a data block is valid.
> + * @rb: The ringbuffer containing the data.
> + * @oldest_data: The oldest data logical position.
> + * @newest_data: The newest data logical position.
> + * @data: The logical position for the data block to check.
> + * @data_next: The logical position for the data block next to this one.
> + *             This value is used to identify the end of the data block.
> + *
> + * A data block is considered valid if it satisfies the two conditions:
> + *
> + * * oldest_data <= data < data_next <= newest_data
> + * * oldest_data is at most exactly 1 wrap behind newest_data
> + *
> + * Return: true if the specified data block is valid.
> + */
> +static inline bool data_valid(struct printk_ringbuffer *rb,
> +			      unsigned long oldest_data,
> +			      unsigned long newest_data,
> +			      unsigned long data, unsigned long data_next)
> +
> +{
> +	return ((data - oldest_data) < DATAARRAY_SIZE(rb) &&
> +		data_next != data &&
> +		(data_next - data) < DATAARRAY_SIZE(rb) &&
> +		(newest_data - data_next) < DATAARRAY_SIZE(rb) &&
> +		(newest_data - oldest_data) <= DATAARRAY_SIZE(rb));

	unsigned long size = DATA_SIZE(rb);

	/* oldest_data <= data */
	if (data - oldest_data >= size);
		return false;

	/* data_next < data */
	if (data_next == data)
		return false

	/* data_next <= newest_data */
	if (newest_data - data_next >= size)
		return false;

	/* 1 wrap */
	if (newest_data - oldest_data >= size)
		return false;

	return true;

> +}
> +
> +/**
> + * add_descr_list() - Add a descriptor to the descriptor list.
> + * @e: An entry that has already reserved data.
> + *
> + * The provided entry contains a pointer to a descriptor that has already
> + * been reserved for this entry. However, the reserved descriptor is not
> + * yet on the list. Add this descriptor as the newest item.
> + *
> + * A descriptor is added in two steps. The first step is to make this
> + * descriptor the newest. The second step is to update the "next" field of
> + * the former newest item to point to this item.
> + */
> +static void add_descr_list(struct prb_reserved_entry *e)
> +{
> +	struct printk_ringbuffer *rb = e->rb;
> +	struct prb_list *l = &rb->descr_list;
> +	struct prb_descr *d = e->descr;
> +	struct prb_descr *newest_d;
> +	unsigned long newest_id;
> +
> +	/* set as newest */
> +	do {
> +		/* MB5: synchronize add descr */
> +		newest_id = smp_load_acquire(&l->newest);
> +		newest_d = TO_DESCR(rb, newest_id);
> +
> +		if (newest_id == EOL)
> +			WRITE_ONCE(d->seq, 1);
> +		else
> +			WRITE_ONCE(d->seq, READ_ONCE(newest_d->seq) + 1);
> +		/*
> +		 * MB5: synchronize add descr
> +		 *
> +		 * In particular: next written before cmpxchg
> +		 */
> +	} while (cmpxchg_release(&l->newest, newest_id, e->id) != newest_id);

What does this pair with? I find ->newest usage in:

  - later this function with an MB6 comment
  - remove_oldest_descr() with no comment
  - expire_oldest_data() with an MB2 comment
  - get_new_lpos() with no comment
  - data_reserve() with an MB2 comment
  - prb_iter_next_valid_entry() with no comment
    (and the smp_rmb()s have no clear comments either).

In short; I've no frigging clue and I might as well just delete all
these comments and reverse engineer :-(

> +
> +	if (unlikely(newest_id == EOL)) {
> +		/* no previous newest means we *are* the list, set oldest */
> +
> +		/*
> +		 * MB UNPAIRED

That's a bug, MB must always be paired.

> +		 *
> +		 * In particular: Force cmpxchg _after_ cmpxchg on newest.
> +		 */
> +		WARN_ON_ONCE(cmpxchg_release(&l->oldest, EOL, e->id) != EOL);
> +	} else {
> +		/* link to previous chain */
> +
> +		/*
> +		 * MB6: synchronize link descr
> +		 *
> +		 * In particular: Force cmpxchg _after_ cmpxchg on newest.

But why... and who cares.

> +		 */
> +		WARN_ON_ONCE(cmpxchg_release(&newest_d->next,
> +					     EOL, e->id) != EOL);
> +	}
> +}