Bug #56791 Remove ios_mutex from InnoDB code

Description:
We have a ios_mutex to protect counter "ios" in io_handler_thread():

/** Count of I/O operations in io_handler_thread() */
static ulint            ios;

io_handler_thread()
{
        for (i = 0;; i++) {
                fil_aio_wait(segment);

                mutex_enter(&ios_mutex);
                ios++;
                mutex_exit(&ios_mutex);
        }

}

This mutex is not necessary. We can make the counter increment atomic, or even if it is not atomic, no need to dedicate a mutex for this counter.

How to repeat:
See the code

Thank you for the bug report.

triage: setting to SRMRTBD (code cleanup)

The variable "ios" that is protected by "ios_mutex" is completely unused. It a static variable in srv0start.c that is only initialized or incremented, never read or copied. We should remove it as well. Because there is some contention on the useless ios_mutex, I think that this bug should be fixed already in 5.5, if not 5.1.

Updated a few counters in the similar situation:

1) Removed ios_mutex
2) Added generic counter operation to increment/decrement following counters atomically if HAVE_ATOMIC_BUILTINS is defined:
 a) ios
 b) dict_table_t::n_foreign_key_checks_running
 c) trx_sys->rseg_history_len

We will fall back to mutex protection for "n_foreign_key_checks_running" and "rseg_history_len" if  HAVE_ATOMIC_BUILTINS not defined.

Fine with the fix in 5.1.

We will use this patch just to remove ios_mutex and ios which is a static counter not being used.

Checked in mysql-5.5-innodb.

Pushed into mysql-trunk 5.6.99-m5 (revid:alexander.nozdrin@oracle.com-20101113155825-czmva9kg4n31anmu) (version source revid:alexander.nozdrin@oracle.com-20101113152450-2zzcm50e7i4j35v7) (merge vers: 5.6.1-m4) (pib:21)

Pushed into mysql-next-mr (revid:alexander.nozdrin@oracle.com-20101113160336-atmtmfb3mzm4pz4i) (version source revid:alexander.nozdrin@oracle.com-20101113152540-gxro4g0v29l27f5x) (pib:21)

Pushed into mysql-5.5 5.5.9 (revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (version source revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (merge vers: 5.5.9) (pib:24)