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c070e5a1ed
SELECT and ALTER TABLE ... REBUILD PARTITION". ALTER TABLE on InnoDB table (including partitioned tables) acquired exclusive locks on rows of table being altered. In cases when there was concurrent transaction which did locking reads from this table this sometimes led to a deadlock which was not detected by MDL subsystem nor by InnoDB engine (and was reported only after exceeding innodb_lock_wait_timeout). This problem stemmed from the fact that ALTER TABLE acquired TL_WRITE_ALLOW_READ lock on table being altered. This lock was interpreted as a write lock and thus for table being altered handler::external_lock() method was called with F_WRLCK as an argument. As result InnoDB engine treated ALTER TABLE as an operation which is going to change data and acquired LOCK_X locks on rows being read from old version of table. In case when there was a transaction which already acquired SR metadata lock on table and some LOCK_S locks on its rows (e.g. by using it in subquery of DML statement) concurrent ALTER TABLE was blocked at the moment when it tried to acquire LOCK_X lock before reading one of these rows. The transaction's attempt to acquire SW metadata lock on table being altered led to deadlock, since it had to wait for ALTER TABLE to release SNW lock. This deadlock was not detected and got resolved only after timeout expiring because waiting were happening in two different subsystems. Similar deadlocks could have occured in other situations. This patch tries to solve the problem by changing ALTER TABLE implementation to use TL_READ_NO_INSERT lock instead of TL_WRITE_ALLOW_READ. After this step handler::external_lock() is called with F_RDLCK as an argument and InnoDB engine correctly interprets ALTER TABLE as operation which only reads data from original version of table. Thanks to this ALTER TABLE acquires only LOCK_S locks on rows it reads. This, in its turn, causes inter-subsystem deadlocks to go away, as all potential lock conflicts and thus deadlocks will be limited to metadata locking subsystem: - When ALTER TABLE reads rows from table being altered it can't encounter any locks which conflict with LOCK_S row locks. There should be no concurrent transactions holding LOCK_X row locks. Such a transaction should have been acquired SW metadata lock on table first which would have conflicted with ALTER's SNW lock. - Vice versa, when DML which runs concurrently with ALTER TABLE tries to lock row it should be requesting only LOCK_S lock which is compatible with locks acquired by ALTER, as otherwise such DML must own an SW metadata lock on table which would be incompatible with ALTER's SNW lock. |
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