[PATCH] copy_process: cleanup bad_fork_cleanup_sighand

The only caller of exit_sighand(tsk) is copy_process's error path.  We can
call __exit_sighand() directly and kill exit_sighand().

This 'tsk' was not yet registered in pid_hash[] or init_task.tasks, it has no
external references, nobody can see it, and

	IF (clone_flags & CLONE_SIGHAND)
		At least 'current' has a reference to ->sighand, this
		means atomic_dec_and_test(sighand->count) can't be true.

	ELSE
		Nobody can see this ->sighand, this means we can free it
		without any locking.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -1151,7 +1151,6 @@ extern void exit_thread(void);
 extern void exit_files(struct task_struct *);
 extern void exit_signal(struct task_struct *);
 extern void __exit_signal(struct task_struct *);
-extern void exit_sighand(struct task_struct *);
 extern void __exit_sighand(struct task_struct *);
 extern void exit_itimers(struct signal_struct *);
 

kernel/fork.c

@@ -1208,7 +1208,8 @@ static task_t *copy_process(unsigned long clone_flags,
 bad_fork_cleanup_signal:
 	exit_signal(p);
 bad_fork_cleanup_sighand:
-	exit_sighand(p);
+	if (p->sighand)
+		__exit_sighand(p);
 bad_fork_cleanup_fs:
 	exit_fs(p); /* blocking */
 bad_fork_cleanup_files:

kernel/signal.c

@@ -336,20 +336,6 @@ void __exit_sighand(struct task_struct *tsk)
 		kmem_cache_free(sighand_cachep, sighand);
 }
 
-void exit_sighand(struct task_struct *tsk)
-{
-	write_lock_irq(&tasklist_lock);
-	rcu_read_lock();
-	if (tsk->sighand != NULL) {
-		struct sighand_struct *sighand = rcu_dereference(tsk->sighand);
-		spin_lock(&sighand->siglock);
-		__exit_sighand(tsk);
-		spin_unlock(&sighand->siglock);
-	}
-	rcu_read_unlock();
-	write_unlock_irq(&tasklist_lock);
-}
-
 /*
  * This function expects the tasklist_lock write-locked.
  */