Messages in this thread | | | Date | Sun, 21 Nov 1999 17:23:05 +0100 | From | Jamie Lokier <> | Subject | Re: [patch] Re: spin_unlock optimization(i386) |
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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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