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Subjectspin_unlock optimization(i386)
the current spin_unlock asm code is
"lock; btrl $0,%0"
it takes ~ 22 ticks on my PII/350.

I think it's possible to replace that with
"movl $0,%0"
which would be a simple, pairable single-tick instruction.

before spin_unlock(), spinlock_t.lock is _always_ 0x0000 0001, ie btrl
is identical to "movl $0,".
--> the only difference is the memory ordering effect of "lock;".

IA32 never reorders write operations, ie even without the "lock;" prefix
spin_unlock() is still a write memory barrier.
Few places will need the read memory barrier, and most of them should
use set_current_state() anyway.

--> What about defining that "spin_unlock()" is only a write memory
barrier and replacing "lock;btrl" with "mov"?

[I guess it's to late to change that for the 2.4 timeframe, but
release_kernel_lock() for i386 could be simplified, perhaps even


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