MDEV-23536 : Race condition between KILL and transaction commit

A race condition may occur between the execution of transaction commit,
and an execution of a KILL statement that would attempt to abort that
transaction.

MDEV-17092 worked around this race condition by modifying InnoDB code.
After that issue was closed, Sergey Vojtovich pointed out that this
race condition would better be fixed above the storage engine layer:

If you look carefully into the above, you can conclude that
thd->free_connection() can be called concurrently with
KILL/thd->awake(). Which is the bug. And it is partially fixed in
THD::~THD(), that is destructor waits for KILL completion:

Fix: Add necessary mutex operations to THD::free_connection()
and move WSREP specific code also there. This ensures that no
one is using THD while we do free_connection(). These mutexes
will also ensures that there can't be concurrent KILL/THD::awake().

innobase_kill_query
  We can now remove usage of trx_sys_mutex introduced on MDEV-17092.

trx_t::free()
  Poison trx->state and trx->mysql_thd

This patch is validated with an RQG run similar to the one that
reproduced MDEV-17092.

Author: Jan Lindström

sql/sql_class.cc

@@ -1357,12 +1357,15 @@ void THD::change_user(void)
 
 /* Do operations that may take a long time */
 
-void THD::cleanup(void)
+void THD::cleanup(bool have_mutex)
 {
   DBUG_ENTER("THD::cleanup");
   DBUG_ASSERT(cleanup_done == 0);
 
-  set_killed(KILL_CONNECTION);
+  if (have_mutex)
+    set_killed_no_mutex(KILL_CONNECTION,0,0);
+  else
+    set_killed(KILL_CONNECTION);
 #ifdef ENABLE_WHEN_BINLOG_WILL_BE_ABLE_TO_PREPARE
   if (transaction.xid_state.xa_state == XA_PREPARED)
   {
@@ -1437,6 +1440,28 @@ void THD::cleanup(void)
 void THD::free_connection()
 {
   DBUG_ASSERT(free_connection_done == 0);
+  /* Check that we have already called thd->unlink() */
+  DBUG_ASSERT(prev == 0 && next == 0);
+
+  /*
+    Other threads may have a lock on THD::LOCK_thd_data or
+    THD::LOCK_thd_kill to ensure that this THD is not deleted
+    while they access it. The following mutex_lock ensures
+    that no one else is using this THD and it's now safe to
+    continue.
+
+    For example consider KILL-statement execution on
+    sql_parse.cc kill_one_thread() that will use
+    THD::LOCK_thd_data to protect victim thread during
+    THD::awake().
+  */
+  mysql_mutex_lock(&LOCK_thd_data);
+  mysql_mutex_lock(&LOCK_thd_kill);
+
+#ifdef WITH_WSREP
+  delete wsrep_rgi;
+  wsrep_rgi= 0;
+#endif /* WITH_WSREP */
   my_free(db);
   db= NULL;
 #ifndef EMBEDDED_LIBRARY
@@ -1445,8 +1470,8 @@ void THD::free_connection()
   net.vio= 0;
   net_end(&net);
 #endif
- if (!cleanup_done)
-   cleanup();
+  if (!cleanup_done)
+    cleanup(true); // We have locked THD::LOCK_thd_kill
   ha_close_connection(this);
   plugin_thdvar_cleanup(this);
   mysql_audit_free_thd(this);
@@ -1457,6 +1482,8 @@ void THD::free_connection()
 #if defined(ENABLED_PROFILING)
   profiling.restart();                          // Reset profiling
 #endif
+  mysql_mutex_unlock(&LOCK_thd_kill);
+  mysql_mutex_unlock(&LOCK_thd_data);
 }
 
 /*
@@ -1512,9 +1539,6 @@ THD::~THD()
   mysql_mutex_lock(&LOCK_thd_data);
   mysql_mutex_unlock(&LOCK_thd_data);
 
-#ifdef WITH_WSREP
-  delete wsrep_rgi;
-#endif
   if (!free_connection_done)
     free_connection();
 

sql/sql_class.h

@@ -3194,7 +3194,7 @@ class THD :public Statement,
   void update_all_stats();
   void update_stats(void);
   void change_user(void);
-  void cleanup(void);
+  void cleanup(bool have_mutex=false);
   void cleanup_after_query();
   void free_connection();
   void reset_for_reuse();

storage/innobase/handler/ha_innodb.cc

@@ -5176,38 +5176,15 @@ static void innobase_kill_query(handlerton*, THD* thd, enum thd_kill_levels)
 
   if (trx_t* trx= thd_to_trx(thd))
   {
+    ut_ad(trx->mysql_thd == thd);
     lock_mutex_enter();
-    trx_sys_mutex_enter();
-    trx_mutex_enter(trx);
-    /* It is possible that innobase_close_connection() is concurrently
-    being executed on our victim. In that case, trx->mysql_thd would
-    be reset before invoking trx_t::free(). Even if the trx object is later
-    reused for another client connection or a background transaction,
-    its trx->mysql_thd will differ from our thd.
-
-    If trx never performed any changes, nothing is really protecting
-    the trx_t::free() call or the changes of trx_t::state when the
-    transaction is being rolled back and trx_commit_low() is being
-    executed.
-
-    The function trx_allocate_for_mysql() acquires
-    trx_sys_t::mutex, but trx_allocate_for_background() will not.
-    Luckily, background transactions cannot be read-only, because
-    for read-only transactions, trx_start_low() will avoid acquiring
-    any of the trx_sys_t::mutex, lock_sys_t::mutex, trx_t::mutex before
-    assigning trx_t::state.
-
-    At this point, trx may have been reallocated for another client
-    connection, or for a background operation. In that case, either
-    trx_t::state or trx_t::mysql_thd should not match our expectations. */
-    bool cancel= trx->mysql_thd == thd && trx->state == TRX_STATE_ACTIVE &&
-      !trx->lock.was_chosen_as_deadlock_victim;
-    trx_sys_mutex_exit();
-    if (!cancel);
-    else if (lock_t *lock= trx->lock.wait_lock)
+    if (lock_t *lock= trx->lock.wait_lock)
+    {
+      trx_mutex_enter(trx);
       lock_cancel_waiting_and_release(lock);
+      trx_mutex_exit(trx);
+    }
     lock_mutex_exit();
-    trx_mutex_exit(trx);
   }
 
   DBUG_VOID_RETURN;

storage/innobase/lock/lock0lock.cc

@@ -6453,6 +6453,7 @@ lock_cancel_waiting_and_release(
 
 	ut_ad(lock_mutex_own());
 	ut_ad(trx_mutex_own(lock->trx));
+	ut_ad(lock->trx->state == TRX_STATE_ACTIVE);
 
 	lock->trx->lock.cancel = true;
 

storage/innobase/trx/trx0trx.cc

@@ -393,7 +393,7 @@ inline void trx_t::free()
   /* do not poison mutex */
   MEM_NOACCESS(&id, sizeof id);
   MEM_NOACCESS(&no, sizeof no);
-  /* state is accessed by innobase_kill_connection() */
+  MEM_NOACCESS(&state, sizeof state);
   MEM_NOACCESS(&is_recovered, sizeof is_recovered);
 #ifdef WITH_WSREP
   MEM_NOACCESS(&wsrep, sizeof wsrep);
@@ -419,7 +419,7 @@ inline void trx_t::free()
   MEM_NOACCESS(&start_time_micro, sizeof start_time_micro);
   MEM_NOACCESS(&commit_lsn, sizeof commit_lsn);
   MEM_NOACCESS(&table_id, sizeof table_id);
-  /* mysql_thd is accessed by innobase_kill_connection() */
+  MEM_NOACCESS(&mysql_thd, sizeof mysql_thd);
   MEM_NOACCESS(&mysql_log_file_name, sizeof mysql_log_file_name);
   MEM_NOACCESS(&mysql_log_offset, sizeof mysql_log_offset);
   MEM_NOACCESS(&n_mysql_tables_in_use, sizeof n_mysql_tables_in_use);