Fix this deadlock - we are already holding the mutex:

=======================================================
[ INFO: possible circular locking dependency detected ] 2.6.38-rc4-test+ #1
-------------------------------------------------------
bash/1850 is trying to acquire lock:
 (text_mutex){+.+.+.}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f

but task is already holding lock:
 (smp_alt){+.+...}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (smp_alt){+.+...}:
       [<ffffffff81082d02>] lock_acquire+0xcd/0xf8
       [<ffffffff8192e119>] __mutex_lock_common+0x4c/0x339
       [<ffffffff8192e4ca>] mutex_lock_nested+0x3e/0x43
       [<ffffffff8101050f>] alternatives_smp_switch+0x77/0x1d8
       [<ffffffff81926a6f>] do_boot_cpu+0xd7/0x762
       [<ffffffff819277dd>] native_cpu_up+0xe6/0x16a
       [<ffffffff81928e28>] _cpu_up+0x9d/0xee
       [<ffffffff81928f4c>] cpu_up+0xd3/0xe7
       [<ffffffff82268d4b>] kernel_init+0xe8/0x20a
       [<ffffffff8100ba24>] kernel_thread_helper+0x4/0x10

-> #1 (cpu_hotplug.lock){+.+.+.}:
       [<ffffffff81082d02>] lock_acquire+0xcd/0xf8
       [<ffffffff8192e119>] __mutex_lock_common+0x4c/0x339
       [<ffffffff8192e4ca>] mutex_lock_nested+0x3e/0x43
       [<ffffffff810568cc>] get_online_cpus+0x41/0x55
       [<ffffffff810a1348>] stop_machine+0x1e/0x3e
       [<ffffffff819314c1>] text_poke_smp_batch+0x3a/0x3c
       [<ffffffff81932b6c>] arch_optimize_kprobes+0x10d/0x11c
       [<ffffffff81933a51>] kprobe_optimizer+0x152/0x222
       [<ffffffff8106bb71>] process_one_work+0x1d3/0x335
       [<ffffffff8106cfae>] worker_thread+0x104/0x1a4
       [<ffffffff810707c4>] kthread+0x9d/0xa5
       [<ffffffff8100ba24>] kernel_thread_helper+0x4/0x10

-> #0 (text_mutex){+.+.+.}:

other info that might help us debug this:

6 locks held by bash/1850:
 #0:  (&buffer->mutex){+.+.+.}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f
 #1:  (s_active#75){.+.+.+}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f
 #2:  (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f
 #3:  (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f
 #4:  (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f
 #5:  (smp_alt){+.+...}, at: [<ffffffff8100a9c1>] return_to_handler+0x0/0x2f

stack backtrace:
Pid: 1850, comm: bash Not tainted 2.6.38-rc4-test+ #1
Call Trace:

 [<ffffffff81080eb2>] print_circular_bug+0xa8/0xb7
 [<ffffffff8192e4ca>] mutex_lock_nested+0x3e/0x43
 [<ffffffff81010302>] alternatives_smp_unlock+0x3d/0x93
 [<ffffffff81010630>] alternatives_smp_switch+0x198/0x1d8
 [<ffffffff8102568a>] native_cpu_die+0x65/0x95
 [<ffffffff818cc4ec>] _cpu_down+0x13e/0x202
 [<ffffffff8117a619>] sysfs_write_file+0x108/0x144
 [<ffffffff8111f5a2>] vfs_write+0xac/0xff
 [<ffffffff8111f7a9>] sys_write+0x4a/0x6e
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: mathieu.desnoyers@efficios.com
Cc: rusty@rustcorp.com.au
Cc: ananth@in.ibm.com
Cc: masami.hiramatsu.pt@hitachi.com
Cc: fweisbec@gmail.com
Cc: jbeulich@novell.com
Cc: jbaron@redhat.com
Cc: mhiramat@redhat.com
LKML-Reference: <1297458466.5226.93.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>