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

Thanks for looking into the patch!

On Thu, Sep 28, 2017 at 3:59 PM, Alexander Korotkov <a.korotkov@postgrespro.ru> wrote:

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.

if xlocked = true, page was already checked for conflict after setting exclusive lock on it's buffer.  I still do not see any problem here...

if (GistFollowRight(stack->page))
{
if (!xlocked)
{
LockBuffer(stack->buffer, GIST_UNLOCK);
LockBuffer(stack->buffer, GIST_EXCLUSIVE);
xlocked = true;
/* someone might've completed the split when we unlocked */
if (!GistFollowRight(stack->page))
continue;