linux-kernel - Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock

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Date:	Mon, 12 Jul 2010 07:45:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	John Kacur <jkacur@...hat.com>, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t

On Sun, 2010-07-11 at 15:33 +0200, Mike Galbraith wrote:
> On Sat, 2010-07-10 at 21:41 +0200, Mike Galbraith wrote:

> diff --git a/kernel/futex.c b/kernel/futex.c
> index a6cec32..ef489f3 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2255,7 +2255,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
>  	/* Queue the futex_q, drop the hb lock, wait for wakeup. */
>  	futex_wait_queue_me(hb, &q, to);
>  
> -	spin_lock(&hb->lock);
> +	/*
> +	 * Non-blocking synchronization point with futex_requeue().
> +	 *
> +	 * We dare not block here because this will alter PI state, possibly
> +	 * before our waker finishes modifying same in wakeup_next_waiter().
> +	 */
> +	while(!spin_trylock(&hb->lock))
> +		cpu_relax();

I agree that this would work. But I wonder if this should have an:

#ifdef PREEMPT_RT
[...]
#else
	spin_lock(&hb->lock);
#endif

around it. Or encapsulate this lock in a macro that does the same thing
(just to keep the actual code cleaner)

-- Steve

>  	ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
>  	spin_unlock(&hb->lock);
>  	if (ret)
> 
> 


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