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 cb4e32fe66b07a7e34efec3e8a1b9bb29b8a02bd..7fd1a4db4ce5b5c4c38e01177263af8180ea4191 100644 (file)
--- a/src/backend/access/gin/ginutil.c
+++ b/src/backend/access/gin/ginutil.c
@@ -228,12 +228,7 @@ 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 1315762ecf970c257ef35f0787c5a0f1d7bdf039..bc10d7b66232142219e2e10ed04a343efb5163d7 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));
 
@@ -710,7 +714,7 @@ ginvacuumcleanup(PG_FUNCTION_ARGS)
                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 53b165326c5ff1cb057c294e729d7c8fee08ab42..e80daf31a9c6dcccf7fa2ce9cff0c22db4bad046 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 fce9e221607ece2a85a98593e518a7a709693190..4ff7c3755c70d69b3b73e70a1b7e27ea66df6190 100644 (file)
--- a/src/include/access/gin_private.h
+++ b/src/include/access/gin_private.h
@@ -13,9 +13,11 @@
 #include "access/genam.h"
 #include "access/gin.h"
 #include "access/itup.h"
+#include "access/transam.h"
 #include "fmgr.h"
-#include "storage/bufmgr.h"
 #include "lib/rbtree.h"
+#include "storage/bufmgr.h"
+#include "utils/snapmgr.h"
 
 
 /*
@@ -131,6 +133,15 @@ 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 +551,7 @@ 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