Re: [patch] Re: Problem with exiting threads under NPTL

[Ingo Molnar <mingo@elte.hu>]

On Sun, 14 Dec 2003, Linus Torvalds wrote:

> Note that both of these micro-optimizations are equally likely to
> _hurt_.

agreed. Patch without these changes reverted is attached. (this is pretty
close to your last patch.)

just one nit wrt. the first micro-optimization:

> So setting variables earlier tends to actually pessimize code [...]

in this specific case the critical section only sets the variable, and the
lifetime of the variable necessiates a stackslot anyway, on x86. So moving
the initialization outside the lock shouldnt pessimise the generated code,
in fact it could even free up a register, due to the compiler being able
to do:

  movl $1, 0x12(%esp)

due to the clear register spill, instead of using up a register due to the
initialization being too close to the setting to 1.
 
But i agree, due to worse readability alone it is not worth having this
change. And on non-x86 it could even result in a spilled register.

(the nonlinear code at the end of the function was clearly a bad idea.)

	Ingo

--- kernel/exit.c.orig
+++ kernel/exit.c
@@ -50,8 +50,10 @@ static void __unhash_process(struct task
 void release_task(struct task_struct * p)
 {
 	task_t *leader;
+	int zap_leader;
 	struct dentry *proc_dentry;
- 
+
+repeat: 
 	BUG_ON(p->state < TASK_ZOMBIE);
  
 	atomic_dec(&p->user->processes);
@@ -70,10 +72,22 @@ void release_task(struct task_struct * p
 	 * group, and the leader is zombie, then notify the
 	 * group leader's parent process. (if it wants notification.)
 	 */
+	zap_leader = 0;
 	leader = p->group_leader;
-	if (leader != p && thread_group_empty(leader) &&
-		    leader->state == TASK_ZOMBIE && leader->exit_signal != -1)
+	if (leader != p && thread_group_empty(leader) && leader->state == TASK_ZOMBIE) {
+		BUG_ON(leader->exit_signal == -1);
 		do_notify_parent(leader, leader->exit_signal);
+		/*
+		 * If we were the last child thread and the leader has
+		 * exited already, and the leader's parent ignores SIGCHLD,
+		 * then we are the one who should release the leader.
+		 *
+		 * do_notify_parent() will have marked it self-reaping in
+		 * that case.
+		 */
+		if (leader->exit_signal == -1)
+			zap_leader = 1;
+	}
 
 	p->parent->cutime += p->utime + p->cutime;
 	p->parent->cstime += p->stime + p->cstime;
@@ -88,6 +102,15 @@ void release_task(struct task_struct * p
 	proc_pid_flush(proc_dentry);
 	release_thread(p);
 	put_task_struct(p);
+
+	/*
+	 * Do this outside the tasklist lock. The reference to the
+	 * leader is safe. There's no recursion possible, since
+	 * the leader of the leader is itself:
+	 */
+	p = leader;
+	if (zap_leader)
+		goto repeat;
 }
 
 /* we are using it only for SMP init */