Re: redundant check in make_data_reusable(): was [PATCH v2 2/3] printk: add lockless buffer

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

On 2020-06-10, Petr Mladek <pmladek@suse.com> wrote:
>> +static bool data_make_reusable(struct printk_ringbuffer *rb,
>> +			       struct prb_data_ring *data_ring,
>> +			       unsigned long lpos_begin,
>> +			       unsigned long lpos_end,
>> +			       unsigned long *lpos_out)
>> +{
>> +	struct prb_desc_ring *desc_ring = &rb->desc_ring;
>> +	struct prb_data_blk_lpos *blk_lpos;
>> +	struct prb_data_block *blk;
>> +	unsigned long tail_lpos;
>> +	enum desc_state d_state;
>> +	struct prb_desc desc;
>> +	unsigned long id;
>> +
>> +	/*
>> +	 * Using the provided @data_ring, point @blk_lpos to the correct
>> +	 * blk_lpos within the local copy of the descriptor.
>> +	 */
>> +	if (data_ring == &rb->text_data_ring)
>> +		blk_lpos = &desc.text_blk_lpos;
>> +	else
>> +		blk_lpos = &desc.dict_blk_lpos;
>> +
>> +	/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
>> +	while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
>> +		blk = to_block(data_ring, lpos_begin);
>> +		id = READ_ONCE(blk->id); /* LMM(data_make_reusable:A) */
>> +
>> +		/*
>> +		 * Guarantee the block ID is loaded before checking the tail
>> +		 * lpos. The loaded block ID can only be considered valid if
>> +		 * the tail lpos has not overtaken @lpos_begin. This pairs
>> +		 * with data_alloc:A.
>> +		 *
>> +		 * Memory barrier involvement:
>> +		 *
>> +		 * If data_make_reusable:A reads from data_alloc:B, then
>> +		 * data_make_reusable:C reads from data_push_tail:D.
>> +		 *
>> +		 * Relies on:
>> +		 *
>> +		 * MB from data_push_tail:D to data_alloc:B
>> +		 *    matching
>> +		 * RMB from data_make_reusable:A to data_make_reusable:C
>> +		 *
>> +		 * Note: data_push_tail:D and data_alloc:B can be different
>> +		 *       CPUs. However, the data_alloc:B CPU (which performs
>> +		 *       the full memory barrier) must have previously seen
>> +		 *       data_push_tail:D.
>> +		 */
>> +		smp_rmb(); /* LMM(data_make_reusable:B) */
>> +
>> +		tail_lpos = atomic_long_read(&data_ring->tail_lpos
>> +					); /* LMM(data_make_reusable:C) */
>> +
>> +		/*
>> +		 * If @lpos_begin has fallen behind the tail lpos, the read
>> +		 * block ID cannot be trusted. Fast forward @lpos_begin to the
>> +		 * tail lpos and try again.
>> +		 */
>> +		if (lpos_begin - tail_lpos >= DATA_SIZE(data_ring)) {
>> +			lpos_begin = tail_lpos;
>> +			continue;
>> +		}
>> +
>> +		d_state = desc_read(desc_ring, id,
>> +				    &desc); /* LMM(data_make_reusable:D) */
>> +
>> +		switch (d_state) {
>> +		case desc_miss:
>> +			return false;
>> +		case desc_reserved:
>> +			return false;
>> +		case desc_committed:
>> +			/*
>> +			 * This data block is invalid if the descriptor
>> +			 * does not point back to it.
>> +			 */
>
> Here again the comments describe what the check does but not why.
> I would write something like:
>
> 			/*
> 			 * The block might have already been
> 			 * reused. Make sure that the descriptor really
> 			 * points back to the checked lpos. It covers
> 			 * both situations. Random data might point to
> 			 * a valid descriptor just by chance. Or the block
> 			 * has been already reused by another descriptor.
> 			 */

Originally this check was needed because the descriptor would be read
even if there was a data race reading the ID from the data
block. Validating the lpos value was a kind of protection against
reading random data that by chance yielded an ID of a committed/reusable
descriptor.

However, after you pointed out that this check was not enough, the code
now re-checks the data tail to make sure that no data race happened. So
actually it is not possible that a descriptor in the committed/reusable
state will point anywhere else. We know the ID is not random garbage or
recycled, so the state can be trusted.

I recommend to either remove this sanity check (for committed and
reusable) or at least change it to:

			WARN_ON_ONCE(blk_lpos->begin != lpos_begin);

Or can you see any possibility of this case?

>> +			if (blk_lpos->begin != lpos_begin)
>> +				return false;
>> +			desc_make_reusable(desc_ring, id);
>> +			break;
>> +		case desc_reusable:
>> +			/*
>> +			 * This data block is invalid if the descriptor
>> +			 * does not point back to it.
>> +			 */
>> +			if (blk_lpos->begin != lpos_begin)
>> +				return false;
>> +			break;
>> +		}
>> +
>> +		/* Advance @lpos_begin to the next data block. */
>> +		lpos_begin = blk_lpos->next;
>> +	}

John Ogness

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