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Commitb899e00

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Fix locking when fixing an incomplete split of a GIN internal page
ginFinishSplit() expects the caller to hold an exclusive lock on thebuffer, but when finishing an earlier "leftover" incomplete split ofan internal page, the caller held a shared lock. That caused anassertion failure in MarkBufferDirty(). Without assertions, it couldlead to corruption if two backends tried to complete the split at thesame time.On master, add a test case using the new injection point facility.Report and analysis by Fei Changhong. Backpatch the fix to allsupported versions.Reviewed-by: Fei Changhong, Michael PaquierDiscussion:https://www.postgresql.org/message-id/tencent_A3CE810F59132D8E230475A5F0F7A08C8307@qq.com
1 parentedbd1b4 commitb899e00

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-18
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1 file changed

+50
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‎src/backend/access/gin/ginbtree.c‎

Lines changed: 50 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -28,6 +28,8 @@ static bool ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
2828
Bufferchildbuf,GinStatsData*buildStats);
2929
staticvoidginFinishSplit(GinBtreebtree,GinBtreeStack*stack,
3030
boolfreestack,GinStatsData*buildStats);
31+
staticvoidginFinishOldSplit(GinBtreebtree,GinBtreeStack*stack,
32+
GinStatsData*buildStats,intaccess);
3133

3234
/*
3335
* Lock buffer by needed method for search.
@@ -109,7 +111,7 @@ ginFindLeafPage(GinBtree btree, bool searchMode,
109111
* encounter on the way.
110112
*/
111113
if (!searchMode&&GinPageIsIncompleteSplit(page))
112-
ginFinishSplit(btree,stack,false,NULL);
114+
ginFinishOldSplit(btree,stack,NULL,access);
113115

114116
/*
115117
* ok, page is correctly locked, we should check to move right ..,
@@ -130,7 +132,7 @@ ginFindLeafPage(GinBtree btree, bool searchMode,
130132
TestForOldSnapshot(snapshot,btree->index,page);
131133

132134
if (!searchMode&&GinPageIsIncompleteSplit(page))
133-
ginFinishSplit(btree,stack,false,NULL);
135+
ginFinishOldSplit(btree,stack,NULL,access);
134136
}
135137

136138
if (GinPageIsLeaf(page))/* we found, return locked page */
@@ -166,8 +168,11 @@ ginFindLeafPage(GinBtree btree, bool searchMode,
166168
* Step right from current page.
167169
*
168170
* The next page is locked first, before releasing the current page. This is
169-
* crucial to protect from concurrent page deletion (see comment in
170-
* ginDeletePage).
171+
* crucial to prevent concurrent VACUUM from deleting a page that we are about
172+
* to step to. (The lock-coupling isn't strictly necessary when we are
173+
* traversing the tree to find an insert location, because page deletion grabs
174+
* a cleanup lock on the root to prevent any concurrent inserts. See Page
175+
* deletion section in the README. But there's no harm in doing it always.)
171176
*/
172177
Buffer
173178
ginStepRight(Bufferbuffer,Relationindex,intlockmode)
@@ -264,7 +269,7 @@ ginFindParents(GinBtree btree, GinBtreeStack *stack)
264269
ptr->parent=root;
265270
ptr->off=InvalidOffsetNumber;
266271

267-
ginFinishSplit(btree,ptr,false,NULL);
272+
ginFinishOldSplit(btree,ptr,NULL,GIN_EXCLUSIVE);
268273
}
269274

270275
leftmostBlkno=btree->getLeftMostChild(btree,page);
@@ -293,7 +298,7 @@ ginFindParents(GinBtree btree, GinBtreeStack *stack)
293298
ptr->parent=root;
294299
ptr->off=InvalidOffsetNumber;
295300

296-
ginFinishSplit(btree,ptr,false,NULL);
301+
ginFinishOldSplit(btree,ptr,NULL,GIN_EXCLUSIVE);
297302
}
298303
}
299304

@@ -674,15 +679,6 @@ ginFinishSplit(GinBtree btree, GinBtreeStack *stack, bool freestack,
674679
booldone;
675680
boolfirst= true;
676681

677-
/*
678-
* freestack == false when we encounter an incompletely split page during
679-
* a scan, while freestack == true is used in the normal scenario that a
680-
* split is finished right after the initial insert.
681-
*/
682-
if (!freestack)
683-
elog(DEBUG1,"finishing incomplete split of block %u in gin index \"%s\"",
684-
stack->blkno,RelationGetRelationName(btree->index));
685-
686682
/* this loop crawls up the stack until the insertion is complete */
687683
do
688684
{
@@ -703,7 +699,7 @@ ginFinishSplit(GinBtree btree, GinBtreeStack *stack, bool freestack,
703699
* would fail.
704700
*/
705701
if (GinPageIsIncompleteSplit(BufferGetPage(parent->buffer)))
706-
ginFinishSplit(btree,parent,false,buildStats);
702+
ginFinishOldSplit(btree,parent,buildStats,GIN_EXCLUSIVE);
707703

708704
/* move right if it's needed */
709705
page=BufferGetPage(parent->buffer);
@@ -727,7 +723,7 @@ ginFinishSplit(GinBtree btree, GinBtreeStack *stack, bool freestack,
727723
page=BufferGetPage(parent->buffer);
728724

729725
if (GinPageIsIncompleteSplit(BufferGetPage(parent->buffer)))
730-
ginFinishSplit(btree,parent,false,buildStats);
726+
ginFinishOldSplit(btree,parent,buildStats,GIN_EXCLUSIVE);
731727
}
732728

733729
/* insert the downlink */
@@ -763,6 +759,42 @@ ginFinishSplit(GinBtree btree, GinBtreeStack *stack, bool freestack,
763759
freeGinBtreeStack(stack);
764760
}
765761

762+
/*
763+
* An entry point to ginFinishSplit() that is used when we stumble upon an
764+
* existing incompletely split page in the tree, as opposed to completing a
765+
* split that we just made outselves. The difference is that stack->buffer may
766+
* be merely share-locked on entry, and will be upgraded to exclusive mode.
767+
*
768+
* Note: Upgrading the lock momentarily releases it. Doing that in a scan
769+
* would not be OK, because a concurrent VACUUM might delete the page while
770+
* we're not holding the lock. It's OK in an insert, though, because VACUUM
771+
* has a different mechanism that prevents it from running concurrently with
772+
* inserts. (Namely, it holds a cleanup lock on the root.)
773+
*/
774+
staticvoid
775+
ginFinishOldSplit(GinBtreebtree,GinBtreeStack*stack,GinStatsData*buildStats,intaccess)
776+
{
777+
elog(DEBUG1,"finishing incomplete split of block %u in gin index \"%s\"",
778+
stack->blkno,RelationGetRelationName(btree->index));
779+
780+
if (access==GIN_SHARE)
781+
{
782+
LockBuffer(stack->buffer,GIN_UNLOCK);
783+
LockBuffer(stack->buffer,GIN_EXCLUSIVE);
784+
785+
if (!GinPageIsIncompleteSplit(BufferGetPage(stack->buffer)))
786+
{
787+
/*
788+
* Someone else already completed the split while we were not
789+
* holding the lock.
790+
*/
791+
return;
792+
}
793+
}
794+
795+
ginFinishSplit(btree,stack, false,buildStats);
796+
}
797+
766798
/*
767799
* Insert a value to tree described by stack.
768800
*
@@ -783,7 +815,7 @@ ginInsertValue(GinBtree btree, GinBtreeStack *stack, void *insertdata,
783815

784816
/* If the leaf page was incompletely split, finish the split first */
785817
if (GinPageIsIncompleteSplit(BufferGetPage(stack->buffer)))
786-
ginFinishSplit(btree,stack,false,buildStats);
818+
ginFinishOldSplit(btree,stack,buildStats,GIN_EXCLUSIVE);
787819

788820
done=ginPlaceToPage(btree,stack,
789821
insertdata,InvalidBlockNumber,

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