exit: __exit_signal: use thread_group_leader() consistently

Cleanup:

- Add the boolean, group_dead = thread_group_leader(), for clarity.

- Do not test/set sig == NULL to detect the all-dead case, use this
  boolean.

- Pass this boolen to __unhash_process() and use it instead of another
  thread_group_leader() call which needs ->group_leader.

  This can be considered as microoptimization, but hopefully this also
  allows us do do other cleanups later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -58,11 +58,11 @@
 
 static void exit_mm(struct task_struct * tsk);
 
-static void __unhash_process(struct task_struct *p)
+static void __unhash_process(struct task_struct *p, bool group_dead)
 {
 	nr_threads--;
 	detach_pid(p, PIDTYPE_PID);
-	if (thread_group_leader(p)) {
+	if (group_dead) {
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
 
@@ -79,6 +79,7 @@ static void __unhash_process(struct task_struct *p)
 static void __exit_signal(struct task_struct *tsk)
 {
 	struct signal_struct *sig = tsk->signal;
+	bool group_dead = thread_group_leader(tsk);
 	struct sighand_struct *sighand;
 	struct tty_struct *uninitialized_var(tty);
 
@@ -92,7 +93,7 @@ static void __exit_signal(struct task_struct *tsk)
 	atomic_dec(&sig->count);
 
 	posix_cpu_timers_exit(tsk);
-	if (thread_group_leader(tsk)) {
+	if (group_dead) {
 		posix_cpu_timers_exit_group(tsk);
 		tty = sig->tty;
 		sig->tty = NULL;
@@ -127,10 +128,9 @@ static void __exit_signal(struct task_struct *tsk)
 		sig->oublock += task_io_get_oublock(tsk);
 		task_io_accounting_add(&sig->ioac, &tsk->ioac);
 		sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
-		sig = NULL; /* Marker for below. */
 	}
 
-	__unhash_process(tsk);
+	__unhash_process(tsk, group_dead);
 
 	/*
 	 * Do this under ->siglock, we can race with another thread
@@ -142,7 +142,7 @@ static void __exit_signal(struct task_struct *tsk)
 
 	__cleanup_sighand(sighand);
 	clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
-	if (sig) {
+	if (group_dead) {
 		flush_sigqueue(&sig->shared_pending);
 		taskstats_tgid_free(sig);
 		tty_kref_put(tty);