MDEV-24643: Assertion failed in rw_lock::update_unlock()

mtr_defer_drop_ahi(): Upgrade the U lock to X lock and downgrade
it back to U lock in case the adaptive hash index needs to be dropped.

This regression was introduced in
commit 03ca6495 (MDEV-24142).

storage/innobase/include/rw_lock.h

 /*****************************************************************************
 
-Copyright (c) 2020, MariaDB Corporation.
+Copyright (c) 2020, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -104,6 +104,14 @@ class rw_lock
     DBUG_ASSERT((l & ~WRITER_WAITING) == UPDATER);
     return true;
   }
+  /** Downgrade an exclusive lock to an update lock. */
+  void downgrade()
+  {
+    IF_DBUG_ASSERT(auto l=,)
+    lock.fetch_xor(WRITER | UPDATER, std::memory_order_relaxed);
+    DBUG_ASSERT((l & ~WRITER_WAITING) == WRITER);
+  }
+
   /** Wait for an exclusive lock.
   @return whether the exclusive lock was acquired */
   bool write_lock_poll()

storage/innobase/include/srw_lock.h

@@ -93,6 +93,7 @@ class ssux_lock_low final : private rw_lock
   void u_lock() { uint32_t l; if (!update_trylock(l)) update_lock(l); }
   bool u_lock_try() { uint32_t l; return update_trylock(l); }
   void u_wr_upgrade() { if (!upgrade_trylock()) write_lock(true); }
+  void wr_u_downgrade() { downgrade(); }
   void wr_lock() { if (!write_trylock()) write_lock(false); }
   void rd_unlock();
   void u_unlock();

storage/innobase/include/sux_lock.h

 /*****************************************************************************
 
-Copyright (c) 2020, MariaDB Corporation.
+Copyright (c) 2020, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -193,6 +193,14 @@ class sux_lock final
   /** Upgrade an update lock */
   inline void u_x_upgrade();
   inline void u_x_upgrade(const char *file, unsigned line);
+  /** Downgrade a single exclusive lock to an update lock */
+  void x_u_downgrade()
+  {
+    ut_ad(have_u_or_x());
+    ut_ad(recursive <= RECURSIVE_MAX);
+    recursive*= RECURSIVE_U;
+    lock.wr_u_downgrade();
+  }
 
   /** Acquire an exclusive lock or upgrade an update lock
   @return whether U locks were upgraded to X */

storage/innobase/mtr/mtr0mtr.cc

@@ -988,13 +988,11 @@ static void mtr_defer_drop_ahi(buf_block_t *block, mtr_memo_type_t fix_type)
     block->lock.s_lock();
     break;
   case MTR_MEMO_PAGE_SX_FIX:
-    block->lock.u_unlock();
-    block->lock.x_lock();
+    block->lock.u_x_upgrade();
     if (dict_index_t *index= block->index)
       if (index->freed())
         btr_search_drop_page_hash_index(block);
-    block->lock.u_lock();
-    block->lock.x_unlock();
+    block->lock.x_u_downgrade();
     break;
   default:
     ut_ad(fix_type == MTR_MEMO_PAGE_X_FIX);