[PATCH 2/5] exec: simplify ->sighand switching

[Oleg Nesterov <oleg@tv-sign.ru>]

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>

--- t/fs/exec.c~2_SWITCH	2007-08-18 18:19:41.000000000 +0400
+++ t/fs/exec.c	2007-08-18 18:26:48.000000000 +0400
@@ -945,12 +945,7 @@ 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);
 
@@ -960,12 +955,11 @@ no_thread_group:
 	BUG_ON(!thread_group_leader(tsk));
 	return 0;
 }
-	
+
 /*
  * 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;