kill_pid_info: don't take now unneeded tasklist_lock

Previously handle_stop_signal(SIGCONT) could drop ->siglock.  That is why
kill_pid_info(SIGCONT) takes tasklist_lock to make sure the target task can't
go away after unlock.  Not needed now.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/signal.h

@@ -362,8 +362,6 @@ int unhandled_signal(struct task_struct *tsk, int sig);
 #define sig_kernel_stop(sig) \
 	(((sig) < SIGRTMIN) && siginmask(sig, SIG_KERNEL_STOP_MASK))
 
-#define sig_needs_tasklist(sig)	((sig) == SIGCONT)
-
 #define sig_user_defined(t, signr) \
 	(((t)->sighand->action[(signr)-1].sa.sa_handler != SIG_DFL) &&	\
 	 ((t)->sighand->action[(signr)-1].sa.sa_handler != SIG_IGN))

kernel/signal.c

@@ -1039,9 +1039,6 @@ int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
 	struct task_struct *p;
 
 	rcu_read_lock();
-	if (unlikely(sig_needs_tasklist(sig)))
-		read_lock(&tasklist_lock);
-
 retry:
 	p = pid_task(pid, PIDTYPE_PID);
 	if (p) {
@@ -1055,10 +1052,8 @@ retry:
 			 */
 			goto retry;
 	}
-
-	if (unlikely(sig_needs_tasklist(sig)))
-		read_unlock(&tasklist_lock);
 	rcu_read_unlock();
+
 	return error;
 }