Re: [RFC] kern: simple futex for gnumach (version 8)

[Marin Ramesa]

On 12/31/2013 04:26:01 PM, Richard Braun wrote:
> On Sun, Dec 29, 2013 at 09:44:51PM +0100, Marin Ramesa wrote:
> > Futex waiters are now in a list and some bugs were fixed.
> >
> > I think this is now ready for test. I have tested this with
> > malloc() and free() instead of kalloc() and kfree(), but
> > without vm_map_lookup() and assert_wait() since these functions
> > segfault on my machine with GDB.
>
> Until you can actually call it from a Hurd userspace program, don't
> consider it ready for test.

I added the RPCs, and am compiling gnumach and glibc on Hurd. I hope
I will be able to do this soon.

> Are you initializing the hash table slots *every* time a futex is
> initialized ??

Yes. I'll fix that. Thanks for noticing.

> Also, where is futex_max_hash_val initialized
> (I mean, to a value different from 0) ?

In the hash function.

> > +int futex_wait(int *address, int value)
> > +{
> > +  struct futex *futex;
> > +  struct futex_waiter waiter;
> > +
> > +  lookup:
> > +
> > +  futex = futex_hash_table_lookup_address(address);
> > +
> > +  if (futex != NULL) {
> > +
> > +          simple_lock(&futex->futex_wait_lock);
> > +
> > +          /* If the value is still the same. */
> > +          if (*address == value) {
> > +                  
> > +                  waiter.thread = current_thread();
> > +                  
> > +                  kern_return_t kr;
> > +			kr = vm_map_lookup(&current_map(),  
> (vm_offset_t)futex->address, VM_PROT_READ, NULL, NULL,
> > +						&waiter.offset, NULL,  
> NULL);
>
> What's the point of this lookup ?

It calculates the offset from the (address, map) value pair.

If you were asking about the hash table lookup. It retreives the
new futex and I use it to check for futex_wait() called with
same address, but different value.

> Also, how do you intend to handle timed waits ?

I haven't thought about this. My first task is to get a working
mutex or a synchronization event out of this. I'll then think
about the timed waits.

> Last, but not least, when are futex objects released ?

When a futex with zero waiters is deleted.

Thanks!