linux-kernel - [PATCH 1/3] wait_task_inactive: "improve" the returned value for ->nvcsw == 0

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Message-ID: <20080730170948.GA18676@tv-sign.ru>
Date:	Wed, 30 Jul 2008 21:09:48 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] wait_task_inactive: "improve" the returned value for ->nvcsw == 0

(on top of wait_task_inactive-dont-consider-task-nivcsw.patch)

wait_task_inactive() returns 1 when p->nvcsw == 0 || p->nvcsw == 1.
This means that two subsequent calls can return the same number while
the task was scheduled in between.

Change the code to return "nvcsw | LONG_MIN" instead of "nvcsw ?: 1",
now the overlap always needs LONG_MAX schedules.

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

--- 27/kernel/sched.c~1_WTI_RET	2008-07-30 19:32:09.000000000 +0400
+++ 27/kernel/sched.c	2008-07-30 19:47:56.000000000 +0400
@@ -1923,7 +1923,7 @@ unsigned long wait_task_inactive(struct 
 		on_rq = p->se.on_rq;
 		ncsw = 0;
 		if (!match_state || p->state == match_state)
-			ncsw = p->nvcsw ?: 1;
+			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
 		task_rq_unlock(rq, &flags);
 
 		/*

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