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Date: Tue, 13 Aug 2002 22:06:04 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [patch] exit_free(), 2.5.31-A0
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> the attached patch implements this (it's ontop of the SETTLS/SETTID
> patch for sched.h conflict reasons), i've tested it with signal 0
> threads and it appears to work fine. (I have not tested kmod yet - that
> comes next.)

as expected, kmod breaks, because it relies on waitpid() working on a
forked child thread, while it blocks SIGCHLD and supresses SIGCHLD
delivery via specifying a 0 exit_signal. No userspace code is expected to
rely on this existing semantics of clone() though, and we should be able
to fix kmod.

a quick workaround is in the attached patch but it's an incorrect fix:  
kmod does not want to receive any signals from the helper thread for a
reason - it can be executed from any process context, correct? I dont know
how this should be fixed.

	Ingo

--- linux/kernel/exit.c.orig	Tue Aug 13 21:01:50 2002
+++ linux/kernel/exit.c	Tue Aug 13 21:49:35 2002
@@ -56,10 +56,13 @@
 
 static void release_task(struct task_struct * p)
 {
-	if (p == current)
+	if (p == current && p->exit_signal)
+		BUG();
+	if (p->state != TASK_ZOMBIE)
 		BUG();
 #ifdef CONFIG_SMP
-	wait_task_inactive(p);
+	if (p != current)
+		wait_task_inactive(p);
 #endif
 	atomic_dec(&p->user->processes);
 	security_ops->task_free_security(p);
@@ -67,10 +70,12 @@
 	unhash_process(p);
 
 	release_thread(p);
-	current->cmin_flt += p->min_flt + p->cmin_flt;
-	current->cmaj_flt += p->maj_flt + p->cmaj_flt;
-	current->cnswap += p->nswap + p->cnswap;
-	sched_exit(p);
+	if (p != current) {
+		current->cmin_flt += p->min_flt + p->cmin_flt;
+		current->cmaj_flt += p->maj_flt + p->cmaj_flt;
+		current->cnswap += p->nswap + p->cnswap;
+		sched_exit(p);
+	}
 	put_task_struct(p);
 }
 
@@ -479,14 +484,15 @@
 
 	write_lock_irq(&tasklist_lock);
 	current->state = TASK_ZOMBIE;
-	do_notify_parent(current, current->exit_signal);
+	if (current->exit_signal)
+		do_notify_parent(current, current->exit_signal);
 	while ((p = eldest_child(current))) {
 		list_del_init(&p->sibling);
 		p->ptrace = 0;
 
 		p->parent = p->real_parent;
 		list_add_tail(&p->sibling,&p->parent->children);
-		if (p->state == TASK_ZOMBIE)
+		if (p->state == TASK_ZOMBIE && p->exit_signal)
 			do_notify_parent(p, p->exit_signal);
 		/*
 		 * process group orphan check
@@ -555,6 +561,9 @@
 
 	tsk->exit_code = code;
 	exit_notify();
+	preempt_disable();
+	if (!current->exit_signal)
+		release_task(current);
 	schedule();
 	BUG();
 /*
--- linux/kernel/signal.c.orig	Tue Aug 13 21:02:04 2002
+++ linux/kernel/signal.c	Tue Aug 13 21:48:01 2002
@@ -768,12 +768,14 @@
 /*
  * Let a parent know about a status change of a child.
  */
-
 void do_notify_parent(struct task_struct *tsk, int sig)
 {
 	struct siginfo info;
 	int why, status;
 
+	if (!sig || !tsk->exit_signal)
+		BUG();
+
 	info.si_signo = sig;
 	info.si_errno = 0;
 	info.si_pid = tsk->pid;
@@ -823,9 +825,11 @@
 void
 notify_parent(struct task_struct *tsk, int sig)
 {
-	read_lock(&tasklist_lock);
-	do_notify_parent(tsk, sig);
-	read_unlock(&tasklist_lock);
+	if (sig != -1) {
+		read_lock(&tasklist_lock);
+		do_notify_parent(tsk, sig);
+		read_unlock(&tasklist_lock);
+	}
 }
 
 #ifndef HAVE_ARCH_GET_SIGNAL_TO_DELIVER
--- linux/kernel/fork.c.orig	Tue Aug 13 21:02:04 2002
+++ linux/kernel/fork.c	Tue Aug 13 21:45:14 2002
@@ -50,6 +50,31 @@
 
 rwlock_t tasklist_lock __cacheline_aligned = RW_LOCK_UNLOCKED;  /* outer */
 
+/*
+ * A per-CPU task cache - this relies on the fact that
+ * the very last portion of sys_exit() is executed with
+ * preemption turned off.
+ */
+static task_t *task_cache[NR_CPUS] __cacheline_aligned;
+
+void __put_task_struct(struct task_struct *tsk)
+{
+	if (tsk != current) {
+		free_thread_info(tsk->thread_info);
+		kmem_cache_free(task_struct_cachep,tsk);
+	} else {
+		int cpu = smp_processor_id();
+
+		tsk = task_cache[cpu];
+		if (tsk) {
+			free_thread_info(tsk->thread_info);
+			kmem_cache_free(task_struct_cachep,tsk);
+		}
+		task_cache[cpu] = current;
+	}
+}
+
+/* Protects next_safe and last_pid. */
 void add_wait_queue(wait_queue_head_t *q, wait_queue_t * wait)
 {
 	unsigned long flags;
@@ -123,13 +148,6 @@
 	return tsk;
 }
 
-void __put_task_struct(struct task_struct *tsk)
-{
-	free_thread_info(tsk->thread_info);
-	kmem_cache_free(task_struct_cachep,tsk);
-}
-
-/* Protects next_safe and last_pid. */
 spinlock_t lastpid_lock = SPIN_LOCK_UNLOCKED;
 
 static int get_pid(unsigned long flags)
--- linux/kernel/kmod.c.orig	Tue Aug 13 22:04:05 2002
+++ linux/kernel/kmod.c	Tue Aug 13 22:04:18 2002
@@ -227,7 +227,7 @@
 		goto out;
 	}
 
-	pid = kernel_thread(exec_modprobe, (void*) module_name, 0);
+	pid = kernel_thread(exec_modprobe, (void*) module_name, SIGCHLD);
 	if (pid < 0) {
 		printk(KERN_ERR "request_module[%s]: fork failed, errno %d\n", module_name, -pid);
 		atomic_dec(&kmod_concurrent);

-
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