[haiku-gsoc] Re: [HCD]: Bfs bug #1

  • From: "Axel Dörfler" <axeld@xxxxxxxxxxxxxxxx>
  • To: haiku-gsoc@xxxxxxxxxxxxx
  • Date: Wed, 18 Jun 2008 11:31:11 +0200 CEST

"Salvatore Benedetto" <emitrax@xxxxxxxxx> wrote:
> it looks like I hit my first bug (#2400) while unpacking a tarball
> containing the whole haiku tree
> (about 480MB compressed) from a usbstick while running in vmware.

Good luck! :-)

> Here is a summary:
> Index::SetTo() is called, it tries to create a Vnode object
> Vnode vnode(fVolume, id); thus with this signature Vnode(Volume
> *volume, ino_t id),
> but then the destructor gets called.
> Actually, now that I look at it, it might fail because the Volume
> constructor fails first.
> I'm not C++ guru, but I thought that if one of the initialization
> would fail, the object wouldn't be created without any needs to call 
> the destructor.

It's allocated on the stack - the constructor cannot fail.

> Anyhow, the ~Vnode gets called, that in turn calls Put()
>         void Put()
>        {
>            if (fVolume)
>                put_vnode(fVolume->FSVolume(), fID);
>            fVolume = NULL;
>        }
> ...
>    dec_vnode_ref_count(vnode, false);

While Get() is called in this case, the destructor would only work 
correctly in case get_vnode() succeeded. Looks like I managed to put a 
bug in that mini class Vnode...
If you want to fix that, please do, otherwise I can fix it on Monday.
The more interesting question would be why get_vnode() failed in the 
first place, though.

> By the way, two things about atomic_add:
> 1. As already stated in the comment above the function, it is in a 
> unusual place
> src/add-ons/kernel/partitioning_systems/intel/intel.cpp

I don't get that. What is where, and what comment describes it?

> 2. How is it atomic? I don't see where the interrupts are disabled.

At least you're not afraid to ask stupid questions ;-)) (sorry, 
couldn't resist :-))
But seriously, if you're interested, have a look at the "Intel IA-32 
Software Developer's Manual".
In short, the "lock" instruction takes care of making this atomic; it's 
the sole purpose of it.


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