Fix locking problem in _hash_squeezebucket() / _hash_freeovflpage().

A bucket squeeze operation needs to lock each page of the bucket
before releasing the prior page, but the previous coding fumbled the
locking when freeing an overflow page during a bucket squeeze
operation.  Commit 6d46f4783efe457f74816a75173eb23ed8930020
introduced this bug.

Amit Kapila, with help from Kuntal Ghosh and Dilip Kumar, after
an initial trouble report by Jeff Janes.  Reviewed by me.  I also
fixed a problem with a comment.

diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index 5f1513bb43c33965096a901fbf91d2a323dbf000..df7838cd6ba8f9d0a8014fa23714c16a4c1f11e4 100644 (file)
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -377,12 +377,11 @@ _hash_firstfreebit(uint32 map)
  * NB: caller must not hold lock on metapage, nor on page, that's next to
  * ovflbuf in the bucket chain.  We don't acquire the lock on page that's
  * prior to ovflbuf in chain if it is same as wbuf because the caller already
- * has a lock on same.  This function releases the lock on wbuf and caller
- * is responsible for releasing the pin on same.
+ * has a lock on same.
  */
 BlockNumber
 _hash_freeovflpage(Relation rel, Buffer ovflbuf, Buffer wbuf,
-                  bool wbuf_dirty, BufferAccessStrategy bstrategy)
+                  BufferAccessStrategy bstrategy)
 {
    HashMetaPage metap;
    Buffer      metabuf;
@@ -447,24 +446,10 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf, Buffer wbuf,
        Assert(prevopaque->hasho_bucket == bucket);
        prevopaque->hasho_nextblkno = nextblkno;
 
+       MarkBufferDirty(prevbuf);
        if (prevblkno != writeblkno)
-       {
-           MarkBufferDirty(prevbuf);
            _hash_relbuf(rel, prevbuf);
-       }
-       else
-       {
-           /* ensure to mark prevbuf as dirty */
-           wbuf_dirty = true;
-       }
    }
-
-   /* write and unlock the write buffer */
-   if (wbuf_dirty)
-       _hash_chgbufaccess(rel, wbuf, HASH_WRITE, HASH_NOLOCK);
-   else
-       _hash_chgbufaccess(rel, wbuf, HASH_READ, HASH_NOLOCK);
-
    if (BlockNumberIsValid(nextblkno))
    {
        Buffer      nextbuf = _hash_getbuf_with_strategy(rel,
@@ -783,30 +768,28 @@ _hash_squeezebucket(Relation rel,
         * Tricky point here: if our read and write pages are adjacent in the
         * bucket chain, our write lock on wbuf will conflict with
         * _hash_freeovflpage's attempt to update the sibling links of the
-        * removed page.  In that case, we don't need to lock it again and we
-        * always release the lock on wbuf in _hash_freeovflpage and then
-        * retake it again here.  This will not only simplify the code, but is
-        * required to atomically log the changes which will be helpful when
-        * we write WAL for hash indexes.
+        * removed page.  In that case, we don't need to lock it again.
         */
        rblkno = ropaque->hasho_prevblkno;
        Assert(BlockNumberIsValid(rblkno));
 
        /* free this overflow page (releases rbuf) */
-       _hash_freeovflpage(rel, rbuf, wbuf, wbuf_dirty, bstrategy);
+       _hash_freeovflpage(rel, rbuf, wbuf, bstrategy);
+
+       if (wbuf_dirty)
+           MarkBufferDirty(wbuf);
 
        /* are we freeing the page adjacent to wbuf? */
        if (rblkno == wblkno)
        {
            /* retain the pin on primary bucket page till end of bucket scan */
-           if (wblkno != bucket_blkno)
-               _hash_dropbuf(rel, wbuf);
+           if (wblkno == bucket_blkno)
+               _hash_chgbufaccess(rel, wbuf, HASH_READ, HASH_NOLOCK);
+           else
+               _hash_relbuf(rel, wbuf);
            return;
        }
 
-       /* lock the overflow page being written, then get the previous one */
-       _hash_chgbufaccess(rel, wbuf, HASH_NOLOCK, HASH_WRITE);
-
        rbuf = _hash_getbuf_with_strategy(rel,
                                          rblkno,
                                          HASH_WRITE,

diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index 9ce44a7f4e13a9e5517b7f1ec4e2ea302adce9ef..627fa2c0c58633a117ddb1317cfaa9e8b6ef5f3e 100644 (file)
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -314,7 +314,7 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 /* hashovfl.c */
 extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf, Buffer wbuf,
-                  bool wbuf_dirty, BufferAccessStrategy bstrategy);
+                  BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
                 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,