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

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Date:	Mon, 12 Jul 2010 14:12:42 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	rostedt@...dmis.org
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 Mon, 2010-07-12 at 07:45 -0400, Steven Rostedt wrote:
> 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)

Yeah, it should.  I'll wait to see what Darren/others say about holding
the wakee's pi_lock across wakeup to plug it.  If he submits something
along that line, I can bin this.

	-Mike

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