exec: simplify ->sighand switching

There is no any reason to do recalc_sigpending() after changing ->sighand.
To begin with, recalc_sigpending() does not take ->sighand into account.

This means we don't need to take newsighand->siglock while changing sighands.
rcu_assign_pointer() provides a necessary barrier, and if another process
reads the new ->sighand it should either take tasklist_lock or it should use
lock_task_sighand() which has a corresponding smp_read_barrier_depends().
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -916,12 +916,7 @@ no_thread_group:
 
 		write_lock_irq(&tasklist_lock);
 		spin_lock(&oldsighand->siglock);
-		spin_lock_nested(&newsighand->siglock, SINGLE_DEPTH_NESTING);
-
 		rcu_assign_pointer(tsk->sighand, newsighand);
-		recalc_sigpending();
-
-		spin_unlock(&newsighand->siglock);
 		spin_unlock(&oldsighand->siglock);
 		write_unlock_irq(&tasklist_lock);
 
@@ -936,7 +931,6 @@ no_thread_group:
  * These functions flushes out all traces of the currently running executable
  * so that a new one can be started
  */
-
 static void flush_old_files(struct files_struct * files)
 {
 	long j = -1;