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

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Date:	Sun, 11 May 2008 07:06:21 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Ingo Molnar <mingo@...e.hu>
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

On Sun, May 11, 2008 at 03:01:27PM +0200, Ingo Molnar wrote:
> * 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?

Still mangled ... and I don't see how we lose a wakeup.  We test for
having the semaphore before we check for having been interrupted, and we
hold the lock the whole time.

IOW, what I think you're checking for is:

task A			task B
			if sem->count >0
				break;
sem->count++
wake_up_process(B)
			if (state == TASK_INTERRUPTIBLE && signal_pending(task))
				break;

which can't happen because of sem->lock.

> 	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);
> +

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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