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------------------------------------------------------------ revno: 2617.68.10 committer: Dmitry Lenev <dlenev@mysql.com> branch nick: mysql-next-bg46673 timestamp: Tue 2009-09-01 19:57:05 +0400 message: Fix for bug #46673 "Deadlock between FLUSH TABLES WITH READ LOCK and DML". Deadlocks occured when one concurrently executed transactions with several statements modifying data and FLUSH TABLES WITH READ LOCK statement or SET READ_ONLY=1 statement. These deadlocks were introduced by the patch for WL 4284: "Transactional DDL locking"/Bug 989: "If DROP TABLE while there's an active transaction, wrong binlog order" which has changed FLUSH TABLES WITH READ LOCK/SET READ_ONLY=1 to wait for pending transactions. What happened was that FLUSH TABLES WITH READ LOCK blocked all further statements changing tables by setting global_read_lock global variable and has started waiting for all pending transactions to complete. Then one of those transactions tried to executed DML, detected that global_read_lock non-zero and tried to wait until global read lock will be released (i.e. global_read_lock becomes 0), indeed, this led to a deadlock. Proper solution for this problem should probably involve full integration of global read lock with metadata locking subsystem (which will allow to implement waiting for pending transactions without blocking DML in them). But since it requires significant changes another, short-term solution for the problem is implemented in this patch. Basically, this patch restores behavior of FLUSH TABLES WITH READ LOCK/ SET READ_ONLY=1 before the patch for WL 4284/bug 989. By ensuring that extra references to TABLE_SHARE are not stored for active metadata locks it changes these statements not to wait for pending transactions. As result deadlock is eliminated. Note that this does not change the fact that active FLUSH TABLES WITH READ LOCK lock or SET READ_ONLY=1 prevent modifications to tables as they also block transaction commits. mysql-test/r/flush_block_commit.result: Adjusted test case after change in FLUSH TABLES WITH READ LOCK behavior - it is no longer blocked by a pending transaction. mysql-test/r/mdl_sync.result: Added test for bug #46673 "Deadlock between FLUSH TABLES WITH READ LOCK and DML". mysql-test/r/read_only_innodb.result: Adjusted test case after change in SET READ_ONLY behavior - it is no longer blocked by a pending transaction. mysql-test/t/flush_block_commit.test: Adjusted test case after change in FLUSH TABLES WITH READ LOCK behavior - it is no longer blocked by a pending transaction. mysql-test/t/mdl_sync.test: Added test for bug #46673 "Deadlock between FLUSH TABLES WITH READ LOCK and DML". mysql-test/t/read_only_innodb.test: Adjusted test case after change in SET READ_ONLY behavior - it is no longer blocked by a pending transaction. sql/sql_base.cc: Disable caching of pointers to TABLE_SHARE objects in MDL subsystem. This means that transactions holding metadata lock on the table will no longer have extra reference to the TABLE_SHARE (due to this lock) and will no longer block concurrent FLUSH TABLES/FLUSH TABLES WITH READ LOCK. Note that this does not change the fact that FLUSH TABLES WITH READ LOCK prevents concurrent transactions from modifying data as it also blocks all commits.
65 lines
1.5 KiB
Text
65 lines
1.5 KiB
Text
# Save the initial number of concurrent sessions
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# Establish connection con1 (user=root)
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# Establish connection con2 (user=root)
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# Establish connection con3 (user=root)
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# Switch to connection con1
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DROP TABLE IF EXISTS t1;
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CREATE TABLE t1 (a INT) ENGINE=innodb;
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BEGIN;
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INSERT INTO t1 VALUES(1);
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# Switch to connection con2
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FLUSH TABLES WITH READ LOCK;
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# Switch to connection con1
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# Sending:
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COMMIT;
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# Switch to connection con2
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# Wait until COMMIT gets blocked.
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# Verify that 'con1' was blocked and data did not move.
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SELECT * FROM t1;
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a
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UNLOCK TABLES;
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# Switch to connection con1
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# Reaping COMMIT
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# Switch to connection con1
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BEGIN;
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SELECT * FROM t1 FOR UPDATE;
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a
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1
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# Switch to connection con2
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BEGIN;
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SELECT * FROM t1 FOR UPDATE;
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# Switch to connection con3
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FLUSH TABLES WITH READ LOCK;
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# Switch to connection con1
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COMMIT;
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# Switch to connection con2
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a
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1
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COMMIT;
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# Switch to connection con3
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UNLOCK TABLES;
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# Switch to connection con2
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COMMIT;
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# Switch to connection con1
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BEGIN;
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INSERT INTO t1 VALUES(10);
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FLUSH TABLES WITH READ LOCK;
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# Switch to connection con2
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FLUSH TABLES WITH READ LOCK;
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UNLOCK TABLES;
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BEGIN;
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SELECT * FROM t1;
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a
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1
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10
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SHOW CREATE DATABASE test;
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Database Create Database
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test CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 */
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COMMIT;
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# Cleanup
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# Switch to connection default and close connections con1, con2, con3
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# We commit open transactions when we disconnect: only then we can
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# drop the table.
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DROP TABLE t1;
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# End of 4.1 tests
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# Wait till all disconnects are completed
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