Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [rfc] SLOB memory ordering issue | Date | Thu, 16 Oct 2008 06:19:53 +1100 |
| |
On Thursday 16 October 2008 05:43, Linus Torvalds wrote: > On Thu, 16 Oct 2008, Nick Piggin wrote: > > Actually, there are surprisingly huge number of them. What I would be > > most comfortable doing, if I was making a kernel to run my life support > > system on an SMP powerpc box, would be to spend zero time on all the > > drivers and whacky things with ctors and just add smp_wmb() after them > > if they are not _totally_ obvious. > > WHY?
I guess I wouldn't bother with your kernel. I was being hypothetical. Can you _prove_ no code has a bug due specifically to this issue?
> THIS HAS NOTHING TO DO WITH CONSTRUCTORS! > > If the driver is using locking, there is no memory ordering issues > what-so-ever. > > And if the driver isn't using locking, IT IS BROKEN.
Did you read the anon_vma example? It's broken if it assumes the objects coming out of its slab are always "stable".
> It's that simple. Why do you keep bringing up non-issues?
Why are you being antagonistic and assuming I'm wrong instead of considering you mistunderstand me, maybe I'm not a retard? I am bad at explaining myself, but I'll try once more.
> What matters is not constructors. Never has been. Constructors are > actually very rare, it's much more common to do > > ptr = kmalloc(..) > .. initialize it by hand .. > > and why do you think constructors are somehow different? They're not.
I think they might be interpreted or viewed by the caller as giving a "stable" object. It is rather more obvious to a caller that it has previous unordered stores if it is doing this ptr = kmalloc(..) .. initialize it by hand ..
I haven't dealt much with constructors myself so I haven't really had to think about it. But I'm sure I could have missed it and been fooled.
If you still don't agree, then fine; if I find a bug I'll send a patch. I don't want to keep arguing.
> What matter is how you look things up on the other CPU's. If you don't use > locking, you use some lockless thing, and then you need to be careful > about memory ordering. > > And quite frankly, if you're a driver, and you're trying to do lockless > algorithms, you're just being crazy. You're going to have much worse bugs, > and again, whether you use constructors or pink elephants is going to be > totally irrelevant. > > So why do you bring up these totally pointless things? Why do you bring up > drivers? Why do you bring up constructors? Why, why, why?
I'll try to keep them to myself in future.
Thanks, Nick
| |