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

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.