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MDEV-7818: Deadlock occurring with parallel replication and FTWRL

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Problem is that FLUSH TABLES WITH READ LOCK first blocks threads from
starting new commits, then waits for running commits to complete. But
in-order parallel replication needs commits to happen in a particular
order, so this can easily deadlock.

To fix this problem, this patch introduces a way to temporarily pause
the parallel replication worker threads. Before starting FTWRL, we let
all worker threads complete in-progress transactions, and then
wait. Then we proceed to take the global read lock. Once the lock is
obtained, we unpause the worker threads. Now commits are blocked from
starting by the global read lock, so the deadlock will no longer occur.
This commit is contained in:
Kristian Nielsen
2015-10-22 11:18:34 +02:00
parent 6d96fab7dd
commit ba02550166
8 changed files with 537 additions and 25 deletions
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13
sql/sql_parse.cc
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@ -4284,6 +4284,17 @@ end_with_restore_list:
break;
}
if (lex->type & REFRESH_READ_LOCK)
{
/*
We need to pause any parallel replication slave workers during FLUSH
TABLES WITH READ LOCK. Otherwise we might cause a deadlock, as
worker threads eun run in arbitrary order but need to commit in a
specific given order.
*/
if (rpl_pause_for_ftwrl(thd))
goto error;
}
/*
reload_acl_and_cache() will tell us if we are allowed to write to the
binlog or not.
@ -4314,6 +4325,8 @@ end_with_restore_list:
if (!res)
my_ok(thd);
}
if (lex->type & REFRESH_READ_LOCK)
rpl_unpause_after_ftwrl(thd);
break;
}