rw_trx_hash_t::find(): Remove redundant code, and avoid committer starvation

1 files changed, 26 insertions, 8 deletions

diff --git a/storage/innobase/include/trx0sys.h b/storage/innobase/include/trx0sys.h
index c92907236e6..20dd14688a5 100644
--- a/storage/innobase/include/trx0sys.h
+++ b/storage/innobase/include/trx0sys.h
@@ -628,12 +628,7 @@ public:
       The caller should already have handled trx_id==0 specially.
     */
     ut_ad(trx_id);
-    if (caller_trx && caller_trx->id == trx_id)
-    {
-      if (do_ref_count)
-        caller_trx->reference();
-      return caller_trx;
-    }
+    ut_ad(!caller_trx || caller_trx->id != trx_id || !do_ref_count);
 
     trx_t *trx= 0;
     LF_PINS *pins= caller_trx ? get_pins(caller_trx) : lf_hash_get_pins(&hash);
@@ -649,8 +644,31 @@ public:
       if ((trx= element->trx)) {
         DBUG_ASSERT(trx_id == trx->id);
         if (do_ref_count)
-          trx->reference();
-        ut_d(validate_element(trx));
+        {
+          TrxMutex *trx_mutex= &trx->mutex;
+          mutex_enter(trx_mutex);
+          ut_ad(!trx_state_eq(trx, TRX_STATE_NOT_STARTED));
+          if (trx_state_eq(trx, TRX_STATE_COMMITTED_IN_MEMORY)) {
+            /* We have an early state check here to avoid committer
+            starvation in a wait loop for transaction references, when
+            there's a stream of trx_sys.find() calls from other
+            threads. The trx->state may change to COMMITTED after
+            trx_mutex is released, and it will have to be rechecked by
+            the caller after reacquiring the mutex. */
+            trx= NULL;
+          }
+          else
+          {
+            /* The reference could be safely incremented after
+            releasing one of trx_mutex or trx_sys->mutex.  Holding
+            trx->mutex here may prevent a few false references that
+            could have a negative performance impact on
+            trx_commit_in_memory(). */
+            trx->reference();
+          }
+          mutex_exit(trx_mutex);
+        }
+        ut_d(if (trx) validate_element(trx));
       }
       mutex_exit(&element->mutex);
     }