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

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Date:	Mon, 26 Oct 2009 12:01:22 -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>
Subject: Re: [patch -rt] Fix infinite loop with 2.6.31.4-rt14 V2

Darren Hart wrote:
> Darren Hart wrote:
>> 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?
> 
> And now that I think about it, when I reviewed this original patch I
> remember mentioning that this isn't even needed for
> futex_wait_requeue_pi() because we don't have the same wake-up race as
> futex_wait() suffers from - since we don't use the same lock_ptr == NULL
> test (nor do we use the wake_list in the requeue code). I suspect the
> only case where -EAGAIN is being used here is when the uval doesn't
> match val - no spurious wakeups.
> 
> Dino, can you try with the following patch which just reverts the
> spurious wakeup handling for the requeue_pi path.


Dino mentioned in IRC that this is basically what he tried originally 
and that it worked fine. Thomas, any objections to this patch?

--
Darren

>  From c21e762bf384e0a559fdf964e0ba7576550d90ec Mon Sep 17 00:00:00 2001
> From: Darren Hart <dvhltc@...ibm.com>
> Date: Fri, 23 Oct 2009 16:18:48 -0700
> Subject: [PATCH] futex: revert spurious wakeup fix for requeue_pi
> 
> The requeue_pi path doesn't use unqueue_me() (and the racy lock_ptr ==
> NULL test) nor does it use the wake_list of futex_wake() which led to
> the following fix.
> 
> 41890f2... futex: Handle spurious wake up
> 
> See debugging discussing on LKML Message-ID: <4AD4080C.20703@...ibm.com>
> 
> The changes in this fix to the requeue_pi path were considered to be a
> likely unecessary, but harmless safety net. Since they are in fact
> causing a problem, just remove them and insert a warning in their place.
> We can remove the warning later, or even in this commit if folks would
> rather.
> 
> Signed-off-by: Darren Hart <dvhltc@...ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> CC: Eric Dumazet <eric.dumazet@...il.com>
> CC: Dinakar Guniguntala <dino@...ibm.com>
> CC: John Stultz <johnstul@...ibm.com>
> 
> Witholding CC to stable for further discussion.
> ---
> kernel/futex.c |   15 +++++++++------
> 1 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 7c4a6ac..7e4e8b2 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2085,12 +2085,19 @@ int handle_early_requeue_pi_wakeup(struct 
> futex_hash_bucket *hb,
>          */
>         plist_del(&q->list, &q->list.plist);
> 
> -        /* Handle spurious wakeups gracefully */
> -        ret = -EAGAIN;
>         if (timeout && !timeout->task)
>             ret = -ETIMEDOUT;
>         else if (signal_pending(current))
>             ret = -ERESTARTNOINTR;
> +        else {
> +            /*
> +             * We don't use the racy unqueue_me() path with the
> +             * q.lock_ptr NULL test, nor does requeue use a
> +             * wake_list. All wakeups here should be accounted for.
> +             */
> +            printk(KERN_ERR "Spurious wakeup in %s\n",
> +                   __FUNCTION__);
> +        }
>     }
>     return ret;
> }
> @@ -2171,7 +2178,6 @@ static int futex_wait_requeue_pi(u32 __user 
> *uaddr, int fshared,
>     q.bitset = bitset;
>     q.rt_waiter = &rt_waiter;
> 
> -retry:
>     key2 = FUTEX_KEY_INIT;
>     ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
>     if (unlikely(ret != 0))
> @@ -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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