Re: [HACKERS] GSoC 2017 : Patch for predicate locking in Gist index

Hi!

On Wed, Jun 21, 2017 at 10:52 AM, Shubham Barai <shubhambaraiss@gmail.com> wrote:

Hi,

On 21 June 2017 at 13:11, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

On 06/16/2017 01:24 PM, Shubham Barai wrote:

@@ -497,6 +499,13 @@ gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
                        for (ptr = dist->next; ptr; ptr = ptr->next)
                                UnlockReleaseBuffer(ptr->buffer);
                }
+
+               for (ptr = dist; ptr; ptr = ptr->next)
+                       PredicateLockPageSplit(rel,
+                                               BufferGetBlockNumber(buffer),
+                                               BufferGetBlockNumber(ptr->buffer));
+
+

I think this new code needs to go before the UnlockReleaseBuffer() calls above. Calling BufferGetBlockNumber() on an already-released buffer is not cool.

- Heikki

I know that. This is the old version of the patch. I had sent updated patch later. Please have a look at updated patch.

I took a look on this patch.

In gistdoinsert() you do CheckForSerializableConflictIn() only if page wasn't exclusively locked before (xlocked is false).

if (!xlocked)
{
LockBuffer(stack->buffer, GIST_UNLOCK);
LockBuffer(stack->buffer, GIST_EXCLUSIVE);
CheckForSerializableConflictIn(r, NULL, stack->buffer);
xlocked = true;

However, page might be exclusively locked before.  And in this case CheckForSerializableConflictIn() would be skipped.  That happens very rarely (someone fixes incomplete split before we did), but nevertheless.