linux-kernel - [PATCH] pi-futex: set PF_EXITING without taking ->pi

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Message-ID: <20070721115712.GA871@tv-sign.ru>
Date:	Sat, 21 Jul 2007 15:57:12 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock

It is a bit annoying that do_exit() takes ->pi_lock to set PF_EXITING.
All we need is to synchronize with lookup_pi_state() which saw this task
without PF_EXITING under ->pi_lock.

Change do_exit() to use spin_unlock_wait().

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

--- t/kernel/exit.c~PF_EXIT	2007-06-19 17:09:15.000000000 +0400
+++ t/kernel/exit.c	2007-07-21 15:34:13.000000000 +0400
@@ -908,13 +908,13 @@ fastcall NORET_TYPE void do_exit(long co
 		schedule();
 	}
 
+	tsk->flags |= PF_EXITING;
 	/*
 	 * tsk->flags are checked in the futex code to protect against
 	 * an exiting task cleaning up the robust pi futexes.
 	 */
-	spin_lock_irq(&tsk->pi_lock);
-	tsk->flags |= PF_EXITING;
-	spin_unlock_irq(&tsk->pi_lock);
+	smp_mb();
+	spin_unlock_wait(&tsk->pi_lock);
 
 	if (unlikely(in_atomic()))
 		printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",

-
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