MDEV-34431: Excessive blocking by mtr_t::index_lock_upgrade()

storage/innobase/btr/btr0cur.cc

@@ -1719,7 +1719,12 @@ ATTRIBUTE_COLD void mtr_t::index_lock_upgrade()
     return;
   ut_ad(slot.type == MTR_MEMO_SX_LOCK);
   index_lock *lock= static_cast<index_lock*>(slot.object);
-  lock->u_x_upgrade(SRW_LOCK_CALL);
+  if (!lock->u_x_upgrade_try())
+  {
+    lock->u_unlock();
+    ut_ad(!lock->have_any());
+    lock->x_lock(SRW_LOCK_CALL);
+  }
   slot.type= MTR_MEMO_X_LOCK;
 }
 

storage/innobase/include/srw_lock.h

@@ -289,6 +289,15 @@ class ssux_lock_impl
 
   bool rd_u_upgrade_try() noexcept { return writer.wr_lock_try(); }
 
+  bool u_wr_upgrade_try() noexcept
+  {
+    DBUG_ASSERT(writer.is_locked());
+    uint32_t lk= 0;
+    return readers.compare_exchange_strong(lk, WRITER,
+                                           std::memory_order_acquire,
+                                           std::memory_order_relaxed);
+  }
+
   void u_wr_upgrade() noexcept
   {
     DBUG_ASSERT(writer.is_locked());
@@ -496,6 +505,7 @@ class ssux_lock
     else
       lock.u_wr_upgrade();
   }
+  bool u_wr_upgrade_try() noexcept { return lock.u_wr_upgrade_try(); }
   bool rd_lock_try() noexcept { return lock.rd_lock_try(); }
   bool u_lock_try() noexcept { return lock.u_lock_try(); }
   bool wr_lock_try() noexcept { return lock.wr_lock_try(); }

storage/innobase/include/sux_lock.h

@@ -196,16 +196,31 @@ class sux_lock final
   /** Upgrade an update lock */
   inline void u_x_upgrade();
   inline void u_x_upgrade(const char *file, unsigned line);
+  /** @return whether an update lock was upgraded to exclusive */
+  bool u_x_upgrade_try() noexcept
+  {
+    ut_ad(have_u_not_x());
+    if (!lock.u_wr_upgrade_try())
+      return false;
+    recursive/= RECURSIVE_U;
+    return true;
+  }
+
   /** @return whether a shared lock was upgraded to exclusive */
-  bool s_x_upgrade_try()
+  bool s_x_upgrade_try() noexcept
   {
     ut_ad(have_s());
     ut_ad(!have_u_or_x());
     if (!lock.rd_u_upgrade_try())
       return false;
     claim_ownership();
     s_unlock();
-    lock.u_wr_upgrade();
+    if (!lock.u_wr_upgrade_try())
+    {
+      ut_d(recursive= RECURSIVE_U);
+      u_s_downgrade();
+      return false;
+    }
     recursive= RECURSIVE_X;
     return true;
   }