signals: send_sig_info: don't take tasklist_lock

The comment in send_sig_info() is wrong, tasklist_lock can't help.

The caller must ensure the task can't go away, otherwise ->sighand can be NULL
even before we take the lock.

p->sighand could be changed by exec(), but I can't imagine how it is possible
to prevent exit(), but not exec().

Since the things seem to work, I assume all callers are correct.  However,
drm_vbl_send_signals() looks broken.  block_all_signals() which is solely used
by drm is definitely broken.
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>

kernel/signal.c

@@ -1138,8 +1138,7 @@ static int kill_something_info(int sig, struct siginfo *info, int pid)
  */
 
 /*
- * These two are the most common entry points.  They send a signal
- * just to the specific thread.
+ * The caller must ensure the task can't exit.
  */
 int
 send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
@@ -1154,17 +1153,9 @@ send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 	if (!valid_signal(sig))
 		return -EINVAL;
 
-	/*
-	 * We need the tasklist lock even for the specific
-	 * thread case (when we don't need to follow the group
-	 * lists) in order to avoid races with "p->sighand"
-	 * going away or changing from under us.
-	 */
-	read_lock(&tasklist_lock);  
 	spin_lock_irqsave(&p->sighand->siglock, flags);
 	ret = specific_send_sig_info(sig, info, p);
 	spin_unlock_irqrestore(&p->sighand->siglock, flags);
-	read_unlock(&tasklist_lock);
 	return ret;
 }