linux-kernel - [PATCH 1/3] set EXIT_DEAD state in do

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Message-ID: <20060826191741.GA157@oleg>
Date:	Sat, 26 Aug 2006 23:17:41 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] set EXIT_DEAD state in do_exit(), not in schedule()

schedule() checks PF_DEAD on every context switch and sets ->state = EXIT_DEAD
to ensure that the exiting task will be deactivated. Note that this EXIT_DEAD
is in fact a "random" value, we can use any bit except normal TASK_XXX values.

It is better to set this state in do_exit() along with PF_DEAD flag and remove
that check in schedule().

We are safe wrt concurrent try_to_wake_up() (for example ptrace, tkill), it can
not change task's ->state: the 'state' argument of try_to_wake_up() can't have
EXIT_DEAD bit. And in case when try_to_wake_up() sees a stale value of ->state
== TASK_RUNNING it will do nothing.

Signed-off-by: Oleg Nesterov <oleg@...sign.ru>

--- 2.6.18-rc4/kernel/exit.c~1_state	2006-08-19 23:47:01.000000000 +0400
+++ 2.6.18-rc4/kernel/exit.c	2006-08-26 20:38:04.000000000 +0400
@@ -957,6 +957,7 @@ fastcall NORET_TYPE void do_exit(long co
 	preempt_disable();
 	BUG_ON(tsk->flags & PF_DEAD);
 	tsk->flags |= PF_DEAD;
+	tsk->state = EXIT_DEAD;

 	schedule();
 	BUG();
--- 2.6.18-rc4/kernel/sched.c~1_state	2006-08-20 00:37:27.000000000 +0400
+++ 2.6.18-rc4/kernel/sched.c	2006-08-26 20:39:02.000000000 +0400
@@ -3311,9 +3311,6 @@ need_resched_nonpreemptible:

 	spin_lock_irq(&rq->lock);

-	if (unlikely(prev->flags & PF_DEAD))
-		prev->state = EXIT_DEAD;
-
 	switch_count = &prev->nivcsw;
 	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
 		switch_count = &prev->nvcsw;

-
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