Fix zeroing of pg_serial page without SLRU bank lock

Bug in commit 53c2a97a9266: we failed to acquire the correct SLRU bank
lock when iterating to zero-out intermediate pages in predicate.c.
Rewrite the code block so that we follow the locking protocol correctly.

Also update an outdated comment in the same file -- SerialSLRULock
exists no more.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Discussion: https://postgr.es/m/2a25eaf4-a3a4-5fd1-6241-9d7c73142085@gmail.com

diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 3f378c0099b35c0ed2021a6c1dac2b60f5bcaaad..d5bbfbd4c6f0ec8cc5420e4dce47efd13f0b61ae 100644 (file)
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -137,7 +137,7 @@
  * SerialControlLock
  *     - Protects SerialControlData members
  *
- * SerialSLRULock
+ * SLRU per-bank locks
  *     - Protects SerialSlruCtl
  *
  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
@@ -908,20 +908,25 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
    if (isNewPage)
        serialControl->headPage = targetPage;
 
-   LWLockAcquire(lock, LW_EXCLUSIVE);
-
    if (isNewPage)
    {
-       /* Initialize intervening pages. */
-       while (firstZeroPage != targetPage)
+       /* Initialize intervening pages; might involve trading locks */
+       for (;;)
        {
-           (void) SimpleLruZeroPage(SerialSlruCtl, firstZeroPage);
+           lock = SimpleLruGetBankLock(SerialSlruCtl, firstZeroPage);
+           LWLockAcquire(lock, LW_EXCLUSIVE);
+           slotno = SimpleLruZeroPage(SerialSlruCtl, firstZeroPage);
+           if (firstZeroPage == targetPage)
+               break;
            firstZeroPage = SerialNextPage(firstZeroPage);
+           LWLockRelease(lock);
        }
-       slotno = SimpleLruZeroPage(SerialSlruCtl, targetPage);
    }
    else
+   {
+       LWLockAcquire(lock, LW_EXCLUSIVE);
        slotno = SimpleLruReadPage(SerialSlruCtl, targetPage, true, xid);
+   }
 
    SerialValue(slotno, xid) = minConflictCommitSeqNo;
    SerialSlruCtl->shared->page_dirty[slotno] = true;