linux-kernel - Re: [git pull] scheduler fixes

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Message-ID: <20080511130127.GA30685@elte.hu>
Date:	Sun, 11 May 2008 15:01:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Sven Wegener <sven.wegener@...aler.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [git pull] scheduler fixes


* Matthew Wilcox <matthew@....cx> wrote:

> +	/* It's possible we need to wake up the next task on the list too */
> +	if (unlikely(sem->count > 1) && !list_empty(&sem->wait_list))
> +		__up(sem);

this needs to check for ret != 0 as well, otherwise we can be woken but 
a timeout can also trigger => we lose a wakeup. I.e. like the patch 
below. Hm?

	Ingo

----------------------------->
Subject: semaphore: fix #3
From: Ingo Molnar <mingo@...e.hu>
Date: Sun May 11 09:51:07 CEST 2008

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/semaphore.c |   24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

Index: linux/kernel/semaphore.c
===================================================================
--- linux.orig/kernel/semaphore.c
+++ linux/kernel/semaphore.c
@@ -194,6 +194,13 @@ struct semaphore_waiter {
 	struct task_struct *task;
 };
 
+static noinline void __sched __up(struct semaphore *sem)
+{
+	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
+						struct semaphore_waiter, list);
+	wake_up_process(waiter->task);
+}
+
 /*
  * Because this function is inlined, the 'state' parameter will be
  * constant, and thus optimised away by the compiler.  Likewise the
@@ -231,6 +238,10 @@ static inline int __sched __down_common(
 	}
 
 	list_del(&waiter.list);
+	if (unlikely(!list_empty(&sem->wait_list)) &&
+					((sem->count > 1) || ret))
+		__up(sem);
+
 	return ret;
 }
 
@@ -254,9 +265,10 @@ static noinline int __sched __down_timeo
 	return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
 }
 
-static noinline void __sched __up(struct semaphore *sem)
-{
-	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
-						struct semaphore_waiter, list);
-	wake_up_process(waiter->task);
-}
+
+	/*
+	 * Rotate sleepers - to make sure all of them get woken in case
+	 * of parallel up()s:
+	 */
+	list_move_tail(&waiter->list, &sem->wait_list);
+
--
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