Re: [rfc] SLOB memory ordering issue

[Nick Piggin <nickpiggin@xxxxxxxxxxxx>]

On Thursday 16 October 2008 05:03, Linus Torvalds wrote:
> On Thu, 16 Oct 2008, Nick Piggin wrote:
> > What do you mean by the allocation is stable?
>
> "all writes done to it before it's exposed".
>
> > 2. I think it could be easy to assume that the allocated object that was
> > initialised with a ctor for us already will have its initializing stores
> > ordered when we get it from slab.
>
> You make tons of assumptions.
>
> You assume that
>  (a) unlocked accesses are the normal case and should be something the
>      allocator should prioritize/care about.
>  (b) that if you have a ctor, it's the only thing the allocator will do.

Yes, as I said, I do not want to add a branch and/or barrier to the
allocator for this. I just want to flag the issue and discuss whether
there is anything that can be done about it.


> I don't think either of those assumptions are at all relevant or
> interesting. Quite the reverse - I'd expect them to be in a very small
> minority.

They will be in the minority or non-existant, but obviously there only
need be one "counterexample" bug to disprove a claim that it never
matters.


> Now, obviously, on pretty much all machines out there (ie x86[-64] and UP
> ARM), smp_wmb() is a no-op, so in that sense we could certainly say that
> "sure, this is a total special case, but we can add a smp_wmb() anyway
> since it won't cost us anything".
>
> On the other hand, on the machines where it doesn't cost us anything, it
> obviously doesn't _do_ anything either, so that argument is pretty
> dubious.
>
> And on machines where the memory ordering _can_ matter, it's going to add
> cost to the wrong point.

When I said "I'd really hate to add a branch to the slab fastpath", it
wasn't a tacit acknowlegement that the barrier is the only way to go,
if it sounded that way.

I meant: I'd *really* hate to add a branch to the slab fastpath :)
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