So if I enable CONFIG_DEBUG_RT_MUTEXES with 2.6.24.7-rt14, I tend to
quickly see a number of BUG warnings when running Java tests:

BUG: jxeinajar/3383: lock count underflow!
Pid: 3383, comm: jxeinajar Not tainted 2.6.24-ibmrt2.5john #3

Call Trace:
 [<ffffffff8107208d>] rt_mutex_deadlock_account_unlock+0x5d/0x70
 [<ffffffff817d6aa5>] rt_read_slowunlock+0x35/0x550
 [<ffffffff8107173d>] rt_mutex_up_read+0x3d/0xc0
 [<ffffffff81072a99>] rt_up_read+0x29/0x30
 [<ffffffff8106e34e>] do_futex+0x32e/0xd40
 [<ffffffff8107173d>] ? rt_mutex_up_read+0x3d/0xc0
 [<ffffffff81072a99>] ? rt_up_read+0x29/0x30
 [<ffffffff8106f370>] compat_sys_futex+0xa0/0x110
 [<ffffffff81010a36>] ? syscall_trace_enter+0x86/0xb0
 [<ffffffff8102ff04>] cstar_do_call+0x1b/0x65

INFO: lockdep is turned off.
---------------------------
| preempt count: 00000001 ]
| 1-level deep critical section nesting:
----------------------------------------
... [<ffffffff817d8e42>] .... __spin_lock_irqsave+0x22/0x60
......[<ffffffff817d6a93>] ..   ( <= rt_read_slowunlock+0x23/0x550)

After some debugging and with Steven's help, we realized that with
rwlocks, rt_mutex_deadlock_account_lock can be called multiple times in
parallel (where as in most cases the mutex must be held by the caller to
to call the function). This can cause integer lock_count value being
used to be non-atomically incremented.

The following patch converts lock_count to a atomic_t and resolves the
warnings.
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Clark Williams <williams@redhat.com>
Cc: dvhltc <dvhltc@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>