linux-kernel - Re: Update futex_q to clarify single waiter symmantics

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Message-ID: <20081218104408.GA14332@elte.hu>
Date:	Thu, 18 Dec 2008 11:44:08 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	"lkml," <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: Update futex_q to clarify single waiter symmantics


* Darren Hart <dvhltc@...ibm.com> wrote:

> From: Darren Hart <dvhltc@...ibm.com>
>
> I've tripped over this a couple times.  The futex_q uses a waiters list
> to represent a single blocked task and then calles wake_up_all().  This
> can lead to confusion in trying to understand the intent of the code,
> which is to have a single futex_q for every task waiting on a futex.
> This patch corrects the problem, using a single pointer to the waiting
> task, and an appropriate call to wake_up, rather than wake_up_all.
>
> Compile and boot tested on an 8way x86_64 machine.
>
> Signed-off-by: Darren Hart <dvhltc@...ibm.com>
> ---
>
> kernel/futex.c |   11 ++++++-----
> 1 files changed, 6 insertions(+), 5 deletions(-)

applied to tip/core/futexes, thanks Darren!

> -	wake_up_all(&q->waiters);
> +	wake_up(&q->waiter);

yeah, the naming and the wake_up_all is a relic from the FUTEX_FD days - 
but we removed that earlier this year:

| commit 82af7aca56c67061420d618cc5a30f0fd4106b80
| Author: Eric Sesterhenn <snakebyte@....de>
| Date:   Fri Jan 25 10:40:46 2008 +0100
|
|     Removal of FUTEX_FD

	Ingo
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