proc: turn signal_struct->count into "int nr_threads"

No functional changes, just s/atomic_t count/int nr_threads/.

With the recent changes this counter has a single user, get_nr_threads()
And, none of its callers need the really accurate number of threads, not
to mention each caller obviously races with fork/exit.  It is only used to
report this value to the user-space, except first_tid() uses it to avoid
the unnecessary while_each_thread() loop in the unlikely case.

It is a bit sad we need a word in struct signal_struct for this, perhaps
we can change get_nr_threads() to approximate the number of threads using
signal->live and kill ->nr_threads later.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/init_task.h

@@ -16,7 +16,7 @@ extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 
 #define INIT_SIGNALS(sig) {						\
-	.count		= ATOMIC_INIT(1), 				\
+	.nr_threads	= 1,						\
 	.wait_chldexit	= __WAIT_QUEUE_HEAD_INITIALIZER(sig.wait_chldexit),\
 	.shared_pending	= { 						\
 		.list = LIST_HEAD_INIT(sig.shared_pending.list),	\

include/linux/sched.h

@@ -527,8 +527,8 @@ struct thread_group_cputimer {
  */
 struct signal_struct {
 	atomic_t		sigcnt;
-	atomic_t		count;
 	atomic_t		live;
+	int			nr_threads;
 
 	wait_queue_head_t	wait_chldexit;	/* for wait4() */
 
@@ -2149,7 +2149,7 @@ extern bool current_is_single_threaded(void);
 
 static inline int get_nr_threads(struct task_struct *tsk)
 {
-	return atomic_read(&tsk->signal->count);
+	return tsk->signal->nr_threads;
 }
 
 /* de_thread depends on thread_group_leader not being a pid based check */

kernel/exit.c

@@ -83,14 +83,10 @@ static void __exit_signal(struct task_struct *tsk)
 	struct sighand_struct *sighand;
 	struct tty_struct *uninitialized_var(tty);
 
-	BUG_ON(!sig);
-	BUG_ON(!atomic_read(&sig->count));
-
 	sighand = rcu_dereference_check(tsk->sighand,
 					rcu_read_lock_held() ||
 					lockdep_tasklist_lock_is_held());
 	spin_lock(&sighand->siglock);
-	atomic_dec(&sig->count);
 
 	posix_cpu_timers_exit(tsk);
 	if (group_dead) {
@@ -130,6 +126,7 @@ static void __exit_signal(struct task_struct *tsk)
 		sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
 	}
 
+	sig->nr_threads--;
 	__unhash_process(tsk, group_dead);
 
 	/*

kernel/fork.c

@@ -877,9 +877,9 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	if (!sig)
 		return -ENOMEM;
 
-	atomic_set(&sig->sigcnt, 1);
-	atomic_set(&sig->count, 1);
+	sig->nr_threads = 1;
 	atomic_set(&sig->live, 1);
+	atomic_set(&sig->sigcnt, 1);
 	init_waitqueue_head(&sig->wait_chldexit);
 	if (clone_flags & CLONE_NEWPID)
 		sig->flags |= SIGNAL_UNKILLABLE;
@@ -1256,9 +1256,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	}
 
 	if (clone_flags & CLONE_THREAD) {
-		atomic_inc(&current->signal->sigcnt);
-		atomic_inc(&current->signal->count);
+		current->signal->nr_threads++;
 		atomic_inc(&current->signal->live);
+		atomic_inc(&current->signal->sigcnt);
 		p->group_leader = current->group_leader;
 		list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);
 	}