Re: [patch] Re: spin_unlock optimization(i386)

Jamie Lokier (lkd@tantalophile.demon.co.uk)
Sun, 21 Nov 1999 17:23:05 +0100

Gerard wrote:
> Even if the simple 'mov' may ensure other processors to have a
> consistent view of the spinlock, it does not prevent the CPU that
> unlocks from playing with speculative execution around the 'unlock'
> and perform speculative reads for example. Without a minimal
> serialization this stuff does not seem safe to me, or at least not for
> ever.

If local speculative reads are a problem, perhaps this would be faster
than "lock; btrl $0,%0":

"movl $0,%0" then rmb().

The rmb() does a locked operation but no cross-CPU traffic on a PPro.
Thus local reads and writes are fully serialised, and bus traffic is
limited to the write which we believe is read correctly by other
processors.

The rmb() can be a separate asm so that GCC can place some register
operations in between if it's that clever (which it isn't, but it might
be one day).

OTOH, I don't see how speculative reads can be a problem. You're
speculatively reading from /outside/ a locked region so anything goes.

-- Jamie

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