Re: Robust futex patch for Linux 2.6.15

Re: Robust futex patch for Linux 2.6.15

[Andrew Morton]

David Singleton <daviado@gmail.com> 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.

Re: Robust futex patch for Linux 2.6.15

[Andi Kleen]

Andrew Morton <akpm@osdl.org> writes:
> 
> +static int futex_deadlock(struct rt_mutex *lock)
> +{
> +	DEFINE_WAIT(wait);
> +
> +	_raw_spin_unlock(&lock->wait_lock);
> +	_raw_spin_unlock(&current->pi_lock);

And why is there a pi_lock if the code isn't supposed to support PI?

-Andi

Re: Robust futex patch for Linux 2.6.15

[Andrew Morton]

Andi Kleen <ak@suse.de> wrote:
>
> Andrew Morton <akpm@osdl.org> writes:
> > 
> > +static int futex_deadlock(struct rt_mutex *lock)
> > +{
> > +	DEFINE_WAIT(wait);
> > +
> > +	_raw_spin_unlock(&lock->wait_lock);
> > +	_raw_spin_unlock(&current->pi_lock);
> 
> And why is there a pi_lock if the code isn't supposed to support PI?
> 

That was a copy-n-paste from a -rt patch I happened to find.  The URL David
sent was bust, and there are various things in the directory there.  Maybe
we were supposed to look in a tarball, dunno.

Re: Robust futex patch for Linux 2.6.15

[Matan Peled]

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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Re: Robust futex patch for Linux 2.6.15

[Andrew Morton]

Matan Peled <chaosite@gmail.com> wrote:
>
> 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?

A file which now appears to have been deleted..

Re [robust-futex-2] : interdiff for memory leak fix

[david singleton]

   Here is an interdiff for a memory leak fix in register_futex.  The 
nice thing
about the slab caches are the ability to spot leaks easily.

diff -u linux-2.6.15/kernel/futex.c linux-2.6.15/kernel/futex.c
--- linux-2.6.15/kernel/futex.c
+++ linux-2.6.15/kernel/futex.c
@@ -1129,6 +1133,8 @@
         if (vma->vm_file && vma->vm_file->f_mapping) {
                 if (vma->vm_file->f_mapping->robust_head == NULL)
                         init_robust_list(vma->vm_file->f_mapping, 
file_futex);
+               else
+                       kmem_cache_free(file_futex_cachep, file_futex);
                 head = 
&vma->vm_file->f_mapping->robust_head->robust_list;
                 sem = &vma->vm_file->f_mapping->robust_head->robust_sem;
         } else {

although Safari seems to always translates tabs to spaces.

David