commit 73b9ebfe126a4a886ee46cbab637374d7024668a
author Oleg Nesterov <oleg@tv-sign.ru> Tue Mar 28 16:11:07 2006 -0800
committer Linus Torvalds <torvalds@g5.osdl.org> Tue Mar 28 18:36:41 2006 -0800
tree d7ba00d4ce76b49c1569334956cd196b35977a04
parent c97d98931ac52ef110b62d9b75c6a6f2bfbc1898

[PATCH] pidhash: don't count idle threads

fork_idle() does unhash_process() just after copy_process().  Contrary,
boot_cpu's idle thread explicitely registers itself for each pid_type with nr
= 0.

copy_process() already checks p->pid != 0 before process_counts++, I think we
can just skip attach_pid() calls and job control inits for idle threads and
kill unhash_process().  We don't need to cleanup ->proc_dentry in fork_idle()
because with this patch idle threads are never hashed in
kernel/pid.c:pid_hash[].

We don't need to hash pid == 0 in pidmap_init().  free_pidmap() is never
called with pid == 0 arg, so it will never be reused.  So it is still possible
to use pid == 0 in any PIDTYPE_xxx namespace from kernel/pid.c's POV.

However with this patch we don't hash pid == 0 for PIDTYPE_PID case.  We still
have have PIDTYPE_PGID/PIDTYPE_SID entries with pid == 0: /sbin/init and
kernel threads which don't call daemonize().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/fork.c

diff --git a/kernel/fork.c b/kernel/fork.c
index 74c6762..0c32e28 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1181,25 +1181,26 @@
 	 */
 	p->ioprio = current->ioprio;
 
-	add_parent(p);
-	if (unlikely(p->ptrace & PT_PTRACED))
-		__ptrace_link(p, current->parent);
+	if (likely(p->pid)) {
+		add_parent(p);
+		if (unlikely(p->ptrace & PT_PTRACED))
+			__ptrace_link(p, current->parent);
 
-	if (thread_group_leader(p)) {
-		p->signal->tty = current->signal->tty;
-		p->signal->pgrp = process_group(current);
-		p->signal->session = current->signal->session;
-		attach_pid(p, PIDTYPE_PGID, process_group(p));
-		attach_pid(p, PIDTYPE_SID, p->signal->session);
+		if (thread_group_leader(p)) {
+			p->signal->tty = current->signal->tty;
+			p->signal->pgrp = process_group(current);
+			p->signal->session = current->signal->session;
+			attach_pid(p, PIDTYPE_PGID, process_group(p));
+			attach_pid(p, PIDTYPE_SID, p->signal->session);
 
-		list_add_tail(&p->tasks, &init_task.tasks);
-		if (p->pid)
+			list_add_tail(&p->tasks, &init_task.tasks);
 			__get_cpu_var(process_counts)++;
+		}
+		attach_pid(p, PIDTYPE_TGID, p->tgid);
+		attach_pid(p, PIDTYPE_PID, p->pid);
+		nr_threads++;
 	}
-	attach_pid(p, PIDTYPE_TGID, p->tgid);
-	attach_pid(p, PIDTYPE_PID, p->pid);
 
-	nr_threads++;
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
@@ -1263,7 +1264,7 @@
 	if (!task)
 		return ERR_PTR(-ENOMEM);
 	init_idle(task, cpu);
-	unhash_process(task);
+
 	return task;
 }