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SubjectRe: SMP spin-locks

Richard B. Johnson writes:
> Spin-locks now transfer control to the .text.lock segment.
> This is a separate segment that can be at an offset that
> is far away from the currently executing code. That may
> cause the cache to be reloaded. Further, each spin-lock
> invocation generates separate code within that segment.
> Question 1: Why?

Because this increases the code density of the common
case, getting the lock immediately.

> Question 2: What is the purpose of the code sequence, "repz nop"
> generated by the spin-lock code? Is this a processor BUG work-around?
> `as` doesn't "like" this sequence and, Intel doesn't seem to
> document it.

It is a hint to the processor that we are executing a spinlock loop
(it does something wrt. keeping the cacheline of the lock in shared
state when possible).

I believe it is documented in the Pentium 4 manuals, previous chips
ignore this sequence and treat it as a pure nop from what I understand.

David S. Miller
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