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Message-ID: <20230516141718.ts6yffyytm2vpwhx@revolver>
Date: Tue, 16 May 2023 10:17:18 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peng Zhang <zhangpeng.00@...edance.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, maple-tree@...ts.infradead.org
Subject: Re: [PATCH 09/10] maple_tree: Rework mas_wr_slot_store() to be
cleaner and more efficient.
* Peng Zhang <zhangpeng.00@...edance.com> [230516 03:27]:
>
>
> 在 2023/5/16 02:01, Liam R. Howlett 写道:
> > * Peng Zhang <zhangpeng.00@...edance.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@...edance.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
> > >
> >
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