In auto-commit mode, updating both trx and non-trx tables (i.e. issuing a mixed
statement) causes the following sequence of events:
1 - "Flush trx changes" (MYSQL_BIN_LOG::write) - T1:
1.1 - mutex_lock (&LOCK_log)
1.2 - mutex_lock (&LOCK_prep_xids)
1.3 - increase prepared_xids
1.4 - mutex_unlock (&LOCK_prep_xids)
1.5 - mutex_unlock (&LOCK_log)
2 - "Flush non-trx changes" (MYSQL_BIN_LOG::write) - T1:
2.1 - mutex_lock (&LOCK_log)
2.2 - mutex_unlock (&LOCK_log)
3. "unlog" - T1
3.1 - mutex_lock (&LOCK_prep_xids)
3.2 - decrease prepared xids
3.3 - pthread_cond_signal(&COND_prep_xids);
3.4 - mutex_unlock (&LOCK_prep_xids)
The "FLUSH logs" command produces the following sequence of events:
1 - "FLUSH logs" command (MYSQL_BIN_LOG::new_file_impl) - user thread:
1.1 - mutex_lock (&LOCK_log)
1.2 - mutex_lock (&LOCK_prep_xids)
1.3 - while (prepared_xids) pthread_cond_wait(..., &LOCK_prep_xids);
1.4 - mutex_unlock (&LOCK_prep_xids)
1.5 - mutex_unlock (&LOCK_log)
A deadlock will arise if T1 flushes the trx changes and thus increases
prepared_xids but before it is able to continue the execution and flush the
non-trx changes, an user thread calls the "FLUSH logs" command and wait that
the prepared_xids is decreased and gets to zero. However, T1 cannot proceed
with the call to "Flush non-trx changes" because it will block in the mutex
"LOCK_log" and by consequence cannot complete the execution and call the
unlog to decrease the prepared_xids.
To fix the problem, we ensure that the non-trx changes are always flushed
before the trx changes.
Note that if you call "Flush non-trx changes" and a concurrent "FLUSH logs" is
issued, the "Flush non-trx changes" may block, but a deadlock will never happen
because the prepared_xids will eventually get to zero. Bottom line, there will
not be any transaction able to increase the prepared_xids because they will
block in the mutex "LOCK_log" (MYSQL_BIN_LOG::write) and those that increased
the prepared_xids will eventually commit and decrease the prepared_xids.
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