linux-kernel - Re: [patch -rt] Fix infinite loop with 2.6.31.4-rt14 V2

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Date:	Fri, 23 Oct 2009 13:41:12 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	dino@...ibm.com
CC:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	John Stultz <johnstul@...ux.vnet.ibm.com>, stable@...nel.org
Subject: Re: [patch -rt] Fix infinite loop with 2.6.31.4-rt14 V2

Dinakar Guniguntala wrote:
 > Application threads calling futex_wait_requeue_pi run in an infinite loop
 > in the kernel if the futex value changes during the call. The following
 > patch fixes the problem.

The key bit here being that EAGAIN == EWOULDBLOCK - who thought that was 
a good idea?

 >
 > Signed-off-by: Dinakar Guniguntala <dino@...ibm.com>
 > Cc: Thomas Gleixner <tglx@...utronix.de>
 > Cc: Darren Hart <dvhltc@...ibm.com>

Signed-off-by: Darren Hart <dvhltc@...ibm.com>

Adding the usual CC list for futexes as well as stable:

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...e.hu>
CC: Eric Dumazet <eric.dumazet@...il.com>
CC: John Stultz <johnstul@...ibm.com>
CC: stable@...nel.org

 >
 > ---
 >  kernel/futex.c |    9 ++++++---
 >  1 file changed, 6 insertions(+), 3 deletions(-)
 >
 > Index: linux-2.6.31.4-rt14-lbf-f1/kernel/futex.c
 > ===================================================================
 > --- linux-2.6.31.4-rt14-lbf-f1.orig/kernel/futex.c
 > +++ linux-2.6.31.4-rt14-lbf-f1/kernel/futex.c
 > @@ -2188,6 +2188,12 @@ retry:
 >  	spin_lock(&hb->lock);
 >  	ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
 >  	spin_unlock(&hb->lock);
 > +	if (ret == -EAGAIN) {
 > +		/* Retry on spurious wakeup */
 > +		put_futex_key(fshared, &q.key);
 > +		put_futex_key(fshared, &key2);
 > +		goto retry;
 > +	}
 >  	if (ret)
 >  		goto out_put_keys;
 >
 > @@ -2264,9 +2270,6 @@ out_put_keys:
 >  out_key2:
 >  	put_futex_key(fshared, &key2);
 >
 > -	/* Spurious wakeup ? */
 > -	if (ret == -EAGAIN)
 > -		goto retry;
 >  out:
 >  	if (to) {
 >  		hrtimer_cancel(&to->timer);


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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