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SubjectRe: [PATCH] x86 bitops.h commentary on instruction reordering
Marcelo Tosatti wrote:
>>>Not really. x86 doesnt have such an instruction.
>>
>>But how then spin_lock() works? It guarantees memory sync between CPUs,
>>doesn't it? Otherwise how can it prevent possible races with concurrent
>>data modifications?
>
> It makes use of the "lock" instruction to lock the bus before the "dec" instruction:
>
> #define spin_lock_string \
> "\n1:\t" \
> "lock ; decb %0\n\t" \
> "js 2f\n" \
>
> That way it prevent races wrt other CPUs. atomic accesses which need to modify
> (atomic_inc, atomic_dec, etc) data also use the "lock" to prevent other CPUs
> from reading the data.
>
> grep for "lock" in include/asm-i386/.
>
> As hpa said, most x86 instructions (except SSE-related ones) are
> strictly ordered (except the cases Alan pointed, which were not known
> to me).
>
> Thats why there is no "sync"-like instruction on x86 (again, except SSE-related
> ones).
>
> This is just a simple and short explanation of how this works. It gets more complex
> you think about cache coherency between processors, etc. For more details
> the best book probably is "UNIX Systems for Modern Architectures.
> Symmetric Multiprocessing and Caching for Kernel Programming" - Curt Schimmel (which
> has been suggested here over and over).

I know basically, how spinlocks work to provide ordering, my question
was about the cache coherency. Thanks for the link to the book, I will
try to find it.

Vlad
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