Messages in this thread | | | Date | Wed, 15 Oct 2008 11:03:08 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [rfc] SLOB memory ordering issue |
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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.
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.
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.
Linus
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