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

John Ogness
2020-06-10T14:57:05Z

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