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

[Darren Hart]

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@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Darren Hart <dvhltc@xxxxxxxxxx>

Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>

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

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
CC: Eric Dumazet <eric.dumazet@xxxxxxxxx>
CC: John Stultz <johnstul@xxxxxxxxxx>
CC: stable@xxxxxxxxxx

>
> ---
>  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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