Re: [PATCH 09/10] maple_tree: Rework mas_wr_slot_store() to be cleaner and more efficient.

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* Peng Zhang <zhangpeng.00@bytedance.com> [230516 03:27]:
> 
> 
> 在 2023/5/16 02:01, Liam R. Howlett 写道:
> > * Peng Zhang <zhangpeng.00@bytedance.com> [230515 09:18]:
> > > The code of mas_wr_slot_store() is messy, make it clearer and concise,
> > > and add comments. In addition, get whether the two gaps are empty to
> > > avoid calling mas_update_gap() all the time.
> > 
> > Please drop the cases from the comments.  These aren't that complicated
> > to need diagrams.
> > 
> > Case 1: Overwriting the range and over a part of the next range
> > Case 2: Overwriting a part of the range and over the entire next range
> > 
> > > 
> > > Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> > > ---
> > >   lib/maple_tree.c | 79 +++++++++++++++++++++++++++---------------------
> > >   1 file changed, 44 insertions(+), 35 deletions(-)
> > > 
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index 538e49feafbe4..d558e7bcb6da8 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -4190,53 +4190,62 @@ static inline bool mas_wr_node_store(struct ma_wr_state *wr_mas)
> > >    * @wr_mas: the maple write state
> > >    *
> > >    * Return: True if stored, false otherwise
> > > + *
> > > + * Case 1:
> > > + *                       r_min   r_max    lmax
> > > + *                 +-------+-------+-------+
> > > + * original range: |       |offset | end   |
> > > + *                 +-----------------------+
> > > + *                         +-----------+
> > > + * overwrite:              |           |
> > > + *                         +-----------+
> > > + *                        index       last
> > > + *
> > > + * Case 2:
> > > + *                       r_min   r_max    lmax
> > > + *                 +-------+-------+-------+
> > > + * original range: |       |offest | end   |
> > > + *                 +-------+---------------+
> > > + *                             +-----------+
> > > + * overwrite:                  |           |
> > > + *                             +-----------+
> > > + *                           index        last
> > >    */
> > >   static inline bool mas_wr_slot_store(struct ma_wr_state *wr_mas)
> > >   {
> > >   	struct ma_state *mas = wr_mas->mas;
> > > -	unsigned long lmax; /* Logical max. */
> > >   	unsigned char offset = mas->offset;
> > > +	unsigned char offset_end = wr_mas->offset_end;
> > > +	unsigned long lmax = wr_mas->end_piv; /* Logical max. */
> > > +	bool gap = false;
> > > -	if ((wr_mas->r_max > mas->last) && ((wr_mas->r_min != mas->index) ||
> > > -				  (offset != wr_mas->node_end)))
> > > -		return false;
> > > -
> > > -	if (offset == wr_mas->node_end - 1)
> > > -		lmax = mas->max;
> > > -	else
> > > -		lmax = wr_mas->pivots[offset + 1];
> > > -
> > > -	/* going to overwrite too many slots. */
> > > -	if (lmax < mas->last)
> > > +	if (offset_end - offset != 1)
> > >   		return false;
> > > -	if (wr_mas->r_min == mas->index) {
> > > -		/* overwriting two or more ranges with one. */
> > > -		if (lmax == mas->last)
> > > -			return false;
> > > -
> > > -		/* Overwriting all of offset and a portion of offset + 1. */
> > > +	if (mas->index == wr_mas->r_min && mas->last < lmax) {
> > > +		/* Case 1 */
> > > +		gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
> > > +		gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
> > >   		rcu_assign_pointer(wr_mas->slots[offset], wr_mas->entry);
> > >   		wr_mas->pivots[offset] = mas->last;
> > > -		goto done;
> > > -	}
> > > -
> > > -	/* Doesn't end on the next range end. */
> > > -	if (lmax != mas->last)
> > > +	} else if (mas->index > wr_mas->r_min && mas->last == lmax) {
> > > +		/* Case 2 */
> > > +		gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset);
> > > +		gap |= !mt_slot_locked(mas->tree, wr_mas->slots, offset + 1);
> > > +		rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
> > > +		wr_mas->pivots[offset] = mas->index - 1;
> > 
> > These two lines need to be in opposite order to ensure a reader sees
> > either the value or the previous value.  If you overwrite something with
> > a new value, it is possible that a reader looking for the next range
> > will get the value stored at offset (but not entry).
> Please think again, did you think wrong?

I did, thanks.

> It doesn't happen, swapping the order introduces the problem.
> If we update the pivot first, it will cause a part of the value
> of the range indexed by offset to change to the value of the
> range indexed by offset+1, which is illegal.
> 
> My assignment order remains the same as the previous version.

Yes, you are correct.  There is a place where the reverse is required
for RCU, but that's appending.

> 
> > 
> > > +		mas->offset++; /* Keep mas accurate. */
> > > +	} else {
> > >   		return false;
> > > +	}
> > > -	/* Overwriting a portion of offset and all of offset + 1 */
> > > -	if ((offset + 1 < mt_pivots[wr_mas->type]) &&
> > > -	    (wr_mas->entry || wr_mas->pivots[offset + 1]))
> > > -		wr_mas->pivots[offset + 1] = mas->last;
> > > -
> > > -	rcu_assign_pointer(wr_mas->slots[offset + 1], wr_mas->entry);
> > > -	wr_mas->pivots[offset] = mas->index - 1;
> > > -	mas->offset++; /* Keep mas accurate. */
> > > -
> > > -done:
> > >   	trace_ma_write(__func__, mas, 0, wr_mas->entry);
> > > -	mas_update_gap(mas);
> > > +	/*
> > > +	 * Only update gap when the new entry is empty or there is an empty
> > > +	 * entry in the original two ranges.
> > > +	 */
> > > +	if (!wr_mas->entry || gap)
> > > +		mas_update_gap(mas);
> > >   	return true;
> > >   }
> > > @@ -4418,7 +4427,7 @@ static inline void mas_wr_modify(struct ma_wr_state *wr_mas)
> > >   	if (new_end == wr_mas->node_end + 1 && mas_wr_append(wr_mas))
> > >   		return;
> > > -	if ((wr_mas->offset_end - mas->offset <= 1) && mas_wr_slot_store(wr_mas))
> > > +	if (new_end == wr_mas->node_end && mas_wr_slot_store(wr_mas))
> > >   		return;
> > >   	else if (mas_wr_node_store(wr_mas))
> > >   		return;
> > > -- 
> > > 2.20.1
> > > 
> >