Re: page table lock patch V15 [0/7]: overview II

On Fri, 14 Jan 2005, Andi Kleen wrote:

> > Are you sure the cmpxchg8b need a lock prefix? Sure it does to
>
> If you want it to be atomic on SMP then yes.
>
> > get the proper "atomic cmpxchg" semantics, but what about a
> > simple 64-bit store... If it boils down to 8 byte load, 8 byte
>
> A 64bit store with a 64bit store instruction is atomic. But
> to do that on 32bit x86 you need SSE/MMX (not an option in the kernel)
> or cmpxchg8
>
> > store on the memory bus, and that store is atomic, then maybe
> > a lock isn't needed at all?
>
> More complex operations than store or load are not atomic without
> LOCK (and not all operations can have a lock prefix). There are a few
> instructions with implicit lock. If you want the gory details read
> chapter 7 in the IA32 Software Developer's Manual Volume 3.

It needs a lock prefix. Volume 2 of the IA32 manual states on page 150
regarding cmpxchg (Note that the atomicity mentioned here seems to apply
to the complete instruction not the 64 bit fetches and stores):


This instruction can be used with a LOCK prefix to allow the instruction
to be executed atomically. To simplify the interface to the processor's
bus, the destination operand receives a write cycle without regard to the
result of the comparison. The destination operand is written back ifthe
comparison fails; otherwise, the source operand is written into the
destination. (The processor never produces a locked read without also
producing a locked write.)
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