Re: Robust futex patch for Linux 2.6.15

[Matan Peled <chaosite@gmail.com>]

Andrew Morton wrote:
> Please send the patch to this mailing list with a full description, as per
> http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt.  And by "full" I
> mean something which tells us what a "robust futex" actually is (it's been
> a year since I thought about them) and why we would want such a thing.
>
> This code looks racy:
>
> +static int futex_deadlock(struct rt_mutex *lock)
> +{
> +	DEFINE_WAIT(wait);
> +
> +	_raw_spin_unlock(&lock->wait_lock);
> +	_raw_spin_unlock(&current->pi_lock);
> +
> +	prepare_to_wait(&deadlocked_futex, &wait, TASK_INTERRUPTIBLE);
> +	schedule();
> +	finish_wait(&deadlocked_futex, &wait);
> +
> +	return -EDEADLK;
> +}
>
> If the spin_unlocks happened after the prepare_to_wait then it would be
> more idoimatic, but without having analysed the wakeup path, I wonder if a
> wakeup which occurs after the spin_unlocks and before the prepare_to_wait()
> will get lost.

Andrew, I'm looking at this:

http://source.mvista.com/~dsingleton/robust-futex-1

And it doesn't seem to have a futex_deadlock function at all. In fact, its seems 
to have a rather lengthy description about robust futexes and why they're a Good 
Thing(TM).

What are you looking at?

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[Name      ]   ::  [Matan I. Peled    ]
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