Memory barriers and spin_unlock safety

[Linus Torvalds]

On Fri, 3 Mar 2006, Benjamin LaHaise wrote:
> 
> Actually, no.  At least in testing an implementation of Dekker's and 
> Peterson's algorithms as a replacement for the locked operation in 
> our spinlocks, it is absolutely necessary to have an sfence in the lock 
> to ensure the lock is visible to the other CPU before proceeding.

I suspect you have some bug in your implementation. I think Dekker's 
algorithm depends on the reads and writes being ordered, and you don't 
seem to do that.

The thing is, you pretty much _have_ to be wrong, because the x86-64 
memory ordering rules are _exactly_ the same as for x86, and we've had 
that simple store as an unlock for a long long time.

		Linus