[PATCH] fix up lockdep trace in fs/exec.c

This fixes the locking error noticed by lockdep:

  =============================================
  [ INFO: possible recursive locking detected ]
  ---------------------------------------------
  init/1 is trying to acquire lock:
   (&sighand->siglock){....}, at: [<c047a78a>] flush_old_exec+0x3ae/0x859

  but task is already holding lock:
   (&sighand->siglock){....}, at: [<c047a77a>] flush_old_exec+0x39e/0x859

  other info that might help us debug this:
  2 locks held by init/1:
   #0:  (tasklist_lock){..--}, at: [<c047a76a>] flush_old_exec+0x38e/0x859
   #1:  (&sighand->siglock){....}, at: [<c047a77a>] flush_old_exec+0x39e/0x859

  stack backtrace:
   [<c04051e1>] show_trace_log_lvl+0x54/0xfd
   [<c040579d>] show_trace+0xd/0x10
   [<c04058b6>] dump_stack+0x19/0x1b
   [<c043b33a>] __lock_acquire+0x773/0x997
   [<c043bacf>] lock_acquire+0x4b/0x6c
   [<c060630b>] _spin_lock+0x19/0x28
   [<c047a78a>] flush_old_exec+0x3ae/0x859
   [<c0498053>] load_elf_binary+0x4aa/0x1628
   [<c0479cab>] search_binary_handler+0xa7/0x24e
   [<c047b577>] do_execve+0x15b/0x1f9
   [<c04022b4>] sys_execve+0x29/0x4d
   [<c0403faf>] syscall_call+0x7/0xb
Signed-off-by: NArjan van de Ven <arjan@infradead.org>
Signed-off-by: NDave Jones <davej@redhat.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -751,7 +751,7 @@ static int de_thread(struct task_struct *tsk)
 
 		write_lock_irq(&tasklist_lock);
 		spin_lock(&oldsighand->siglock);
-		spin_lock(&newsighand->siglock);
+		spin_lock_nested(&newsighand->siglock, SINGLE_DEPTH_NESTING);
 
 		rcu_assign_pointer(current->sighand, newsighand);
 		recalc_sigpending();