Prevent GIN deleted pages from being reclaimed too early

When GIN vacuum deletes a posting tree page, it assumes that no concurrent
searchers can access it, thanks to ginStepRight() locking two pages at once.
However, since 9.4 searches can skip parts of posting trees descending from the
root.  That leads to the risk that page is deleted and reclaimed before
concurrent search can access it.

This commit prevents the risk of above by waiting for every transaction, which
might wait to reference this page, to finish.  Due to binary compatibility
we can't change GinPageOpaqueData to store corresponding transaction id.
Instead we reuse page header pd_prune_xid field, which is unused in index pages.

Discussion: https://postgr.es/m/31a702a.14dd.166c1366ac1.Coremail.chjischj%40163.com
Author: Andrey Borodin, Alexander Korotkov
Reviewed-by: Alexander Korotkov
Backpatch-through: 9.4

diff --git a/src/backend/access/gin/README b/src/backend/access/gin/README
index fade0cbb617103c52000afa8f1402b09467febc6..d551df1166c27c1cef548f493aaf87da3eaab74e 100644 (file)
--- a/src/backend/access/gin/README
+++ b/src/backend/access/gin/README
@@ -304,12 +304,10 @@ the lock on next page has been acquired.
 The downlink is more tricky. A search descending the tree must release the
 lock on the parent page before locking the child, or it could deadlock with
 a concurrent split of the child page; a page split locks the parent, while
-already holding a lock on the child page. However, posting trees are only
-fully searched from left to right, starting from the leftmost leaf. (The
-tree-structure is only needed by insertions, to quickly find the correct
-insert location). So as long as we don't delete the leftmost page on each
-level, a search can never follow a downlink to page that's about to be
-deleted.
+already holding a lock on the child page. So, deleted page cannot be reclaimed
+immediately. Instead, we have to wait for every transaction, which might wait
+to reference this page, to finish. Corresponding processes must observe that
+the page is marked deleted and recover accordingly.
 
 The previous paragraph's reasoning only applies to searches, and only to
 posting trees. To protect from inserters following a downlink to a deleted

diff --git a/src/backend/access/gin/ginutil.c b/src/backend/access/gin/ginutil.c
index d9146488c4450c6828fd4df1f3348f8fd024d3ed..637caf9fdcf6901ab78e7951eb7b4f7d7ffeef5c 100644 (file)
--- a/src/backend/access/gin/ginutil.c
+++ b/src/backend/access/gin/ginutil.c
@@ -274,12 +274,7 @@ GinNewBuffer(Relation index)
                 */
                if (ConditionalLockBuffer(buffer))
                {
-                       Page            page = BufferGetPage(buffer);
-
-                       if (PageIsNew(page))
-                               return buffer;  /* OK to use, if never initialized */
-
-                       if (GinPageIsDeleted(page))
+                       if (GinPageIsRecyclable(BufferGetPage(buffer)))
                                return buffer;  /* OK to use */
 
                        LockBuffer(buffer, GIN_UNLOCK);

diff --git a/src/backend/access/gin/ginvacuum.c b/src/backend/access/gin/ginvacuum.c
index 3d23cebf6283b78e20e9cbd7213d242fe0b63834..58f0d03fd1e88b9f15acd5fa0a1adf2fde7d5e96 100644 (file)
--- a/src/backend/access/gin/ginvacuum.c
+++ b/src/backend/access/gin/ginvacuum.c
@@ -215,6 +215,9 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
        page = BufferGetPage(dBuffer);
        rightlink = GinPageGetOpaque(page)->rightlink;
 
+       /* For deleted page remember last xid which could knew its address */
+       GinPageSetDeleteXid(page, ReadNewTransactionId());
+
        page = BufferGetPage(lBuffer);
        GinPageGetOpaque(page)->rightlink = rightlink;
 
@@ -262,6 +265,7 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 
                data.parentOffset = myoff;
                data.rightLink = GinPageGetOpaque(page)->rightlink;
+               data.deleteXid = GinPageGetDeleteXid(page);
 
                XLogRegisterData((char *) &data, sizeof(ginxlogDeletePage));
 
@@ -708,7 +712,7 @@ ginvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
                LockBuffer(buffer, GIN_SHARE);
                page = (Page) BufferGetPage(buffer);
 
-               if (PageIsNew(page) || GinPageIsDeleted(page))
+               if (GinPageIsRecyclable(page))
                {
                        Assert(blkno != GIN_ROOT_BLKNO);
                        RecordFreeIndexPage(index, blkno);

diff --git a/src/backend/access/gin/ginxlog.c b/src/backend/access/gin/ginxlog.c
index 19ab1b61dce52e7b5b4a72f4fd5bee46d3199756..bfe88827e4cb0ef4305d9146a150ef73e08f3f9e 100644 (file)
--- a/src/backend/access/gin/ginxlog.c
+++ b/src/backend/access/gin/ginxlog.c
@@ -529,6 +529,7 @@ ginRedoDeletePage(XLogReaderState *record)
                page = BufferGetPage(dbuffer);
                Assert(GinPageIsData(page));
                GinPageGetOpaque(page)->flags = GIN_DELETED;
+               GinPageSetDeleteXid(page, data->deleteXid);
                PageSetLSN(page, lsn);
                MarkBufferDirty(dbuffer);
        }

diff --git a/src/include/access/gin_private.h b/src/include/access/gin_private.h
index 0b6006f0ad49d7e73005fa2d3ca0e4c2e8d6375a..8ee17b72a5974067cf66fd59931012c41848c287 100644 (file)
--- a/src/include/access/gin_private.h
+++ b/src/include/access/gin_private.h
@@ -13,6 +13,7 @@
 #include "access/amapi.h"
 #include "access/gin.h"
 #include "access/itup.h"
+#include "access/transam.h"
 #include "fmgr.h"
 #include "storage/bufmgr.h"
 #include "lib/rbtree.h"
@@ -131,6 +132,15 @@ typedef struct GinMetaPageData
 
 #define GinPageRightMost(page) ( GinPageGetOpaque(page)->rightlink == InvalidBlockNumber)
 
+/*
+ * We should reclaim deleted page only once every transaction started before
+ * its deletion is over.
+ */
+#define GinPageGetDeleteXid(page) ( ((PageHeader) (page))->pd_prune_xid )
+#define GinPageSetDeleteXid(page, xid) ( ((PageHeader) (page))->pd_prune_xid = xid)
+#define GinPageIsRecyclable(page) ( PageIsNew(page) || (GinPageIsDeleted(page) \
+       && TransactionIdPrecedes(GinPageGetDeleteXid(page), RecentGlobalXmin)))
+
 /*
  * We use our own ItemPointerGet(BlockNumber|OffsetNumber)
  * to avoid Asserts, since sometimes the ip_posid isn't "valid"
@@ -540,6 +550,7 @@ typedef struct ginxlogDeletePage
 {
        OffsetNumber parentOffset;
        BlockNumber rightLink;
+       TransactionId deleteXid;        /* last Xid which could see this page in scan */
 } ginxlogDeletePage;
 
 #define XLOG_GIN_UPDATE_META_PAGE 0x60