Re: Robust futex patch for Linux 2.6.15

[Andrew Morton]

David Singleton <daviado@xxxxxxxxx> wrote:
>
> Andrew and Ingo,
> 
>      here is a patchthat I'd like to see tested in the mm kernel.  The patch
>  supports robust futexes for Linux without any RT support.
> Ulrich Drepper has been asking me for a while for a patch that just has
> robustness
> in it, no RT or PI or PQ.   He'd like to see it in Linux and said he'd
> support
> it in glibc if/when it gets in.
> 
>      This patch was originally done by Todd Kneisel for the robust-mutex SIG
> at
> OSDL.  I've fixed a few bugs and added slab support.
> 
>      The patch is at
> 
>       http://source.mvista.com/~dsingleton/patch-2.6.15-robust-futex-1
> 
>      There are also some simple tests for robustness in the same directory
>       in robust-tests.tar.gz.  These simple tests test register, deregister,
> waiting,
>       timed waiting,  waiting for robustness from a dieing thread to wake,
> etc.
> 

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.
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