MySQL Bugs: #68876: Disabling innodb_thread_concurrency might cause queries to hang

Bug #68876	Disabling innodb_thread_concurrency might cause queries to hang
Submitted:	5 Apr 2013 21:53	Modified:	9 Apr 2013 5:32
Reporter:	Davi Arnaut (OCA)	Email Updates:
Status:	Verified	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S2 (Serious)
Version:	5.5.30	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	concurrency tickets, innodb, thread concurrency

Description:
Disabling InnoDB's thread concurrency at runtime causes threads waiting in the concurrency queue to hang. The problem is that once innodb_thread_concurrency is set to 0, threads waiting in the queue are do not waked up, neither when the variable is set to zero, nor when threads that were inside InnoDB exit.

How to repeat:
CREATE TABLE t1 (a INT PRIMARY KEY, b INT) ENGINE=InnoDB;
INSERT INTO t1 VALUES (1,1),(2,2),(3,3);

SET GLOBAL innodb_thread_concurrency = 1;

connect(con1,localhost,root,,);
BEGIN;
SELECT * FROM t1 WHERE a = 1;

connect(con2,localhost,root,,);
BEGIN;
SELECT * FROM t1 WHERE a = 1;

connection con1;
SET DEBUG_SYNC='innodb_row_update_for_mysql_begin SIGNAL inside_innodb WAIT_FOR go';
send UPDATE t1 SET b = b + 1 WHERE a = 2;

connection default;
SET DEBUG_SYNC='now WAIT_FOR inside_innodb';

connection con2;
send SELECT * FROM t1 WHERE a = 3;

connection default;
let $wait_condition=
  SELECT COUNT(*) = 1 FROM INFORMATION_SCHEMA.INNODB_TRX
  WHERE trx_query = 'SELECT * FROM t1 WHERE a = 3' AND
        trx_operation_state = 'waiting in InnoDB queue';
--source include/wait_condition.inc

SET GLOBAL innodb_thread_concurrency = 0;

SET DEBUG_SYNC='now SIGNAL go';

--echo # reap con1
connection con1;
reap;

--echo # reap con2
connection con2;
reap;

I thought Domas reported this long ago?

There is Bug#40760, but that is supposedly fixed in 5.5.

Bug #69810 Marked as duplicate of this