Re: spin_unlock optimization(i386)
Manfred Spraul (manfreds@colorfullife.com)
Sun, 21 Nov 1999 08:36:58 +0100
"Richard B. Johnson" wrote:
>
> On Sat, 20 Nov 1999, Manfred Spraul wrote:
>
> > 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.
>
> Erm.... What about SMP machines? Are you going to get rid of them?
> The purpose of the lock prefix is not to make the current CPU operation
> atomic. It's to make all other CPUs halt until the operation is complete.
> This gurantees that only one CPU modifies the variable at the same
> time. You are not going to do that with a move.
>
I'm only talking about the _unlock_, obviously spin_lock() must use the
lock
prefix, and spin_lock needs a full memory barrier:
unlock means that you own the lock, and that all other CPUs are waiting,
and: we know the current value of spinlock_t.lock:
lock;btrl means
cpu pull LOCK
cpu reads spinlock_t.lock (always 0x0000 0001)
cpu clears bit 0, and updates the carry flag
cpu writes spinlock_t.lock (always 0x0000 0000)
cpu releases LOCK
mov means
cpu writes spinlock_t.lock (always 0x0)
memory writes to 32-bit values are always atomic (and atomic_set()
relies on this)
--> the only difference is the memory ordering.
--
Manfred
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