Re: [PATCH 2.6.13] lockless pagecache 2/7

Nick Piggin
2005-09-02T21:43:38Z

Bear with me Dave, I'll repeat myself a bit, for the benefit of lkml.

Andi Kleen wrote:
>>>Yeah quite a few. I suspect most MIPS also would have a problem in this
>>>area.
>>
>>cmpxchg can be done with LL/SC can't it? Any MIPS should have that.
>
>
> Right.
>
> On PARISC, I don't see where they are emulating compare and swap
> as indicated. They are doing the funny hashed spinlocks for the
> atomic_t operations and bitops, but that is entirely different.
>

Yep, same as SPARC (at least, SPARC's 32-bit atomic_t).

> cmpxchg() has to operate in an environment where, unlike the atomic_t
> and bitops, you cannot control the accessors to the object at all.
>
> The DRM is the only place in the kernel that requires cmpxchg()
> and you can thus make a list of what platform can provide cmpxchg()
> by which ones support DRM and thus provide the cmpxchg() macro already
> in asm/system.h
>
> We really can't require support for this primitive kernel wide, it's
> simply not possible on a couple chips.

Not a generic cmpxchg, no. However, I _believe_ that those
architectures that are missing something like ll/sc or real
atomic cmpxchg should still be able to implement an
"atomic_cmpxchg" on their atomic type.

Sorry if I wasn't at all clear initially. What I'd be interested
in is an architecture that doesn't support ll/sc or real cmpxchg
*and* does not implement atomic_t operations with locks.

Thanks,
Nick

--
SUSE Labs, Novell Inc.

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