Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [rfc] SLOB memory ordering issue | Date | Thu, 16 Oct 2008 05:12:28 +1100 |
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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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