Re: SMP spin-locks

[David S. Miller]

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.

Later,
David S. Miller
[e-mail]