On 11/20, Alan Stern wrote:
>
> On Mon, 20 Nov 2006, Oleg Nesterov wrote:
>
> > So, if we have global A == B == 0,
> >
> > CPU_0 CPU_1
> >
> > A = 1; B = 2;
> > mb(); mb();
> > b = B; a = A;
> >
> > It could happen that a == b == 0, yes?
>
> Both CPUs execute their "mb" instructions. The mb forces each
> cache to wait until it receives an Acknowdgement for the
> Invalidate it sent.
>
> Both caches send an Acknowledgement message to the other. The
> mb instructions complete.
>
> "b = B" and "a = A" execute. The caches return A==0 and B==0
> because they haven't yet invalidated their cache lines.
>
> The reason the code failed is because the mb instructions didn't force
> the caches to wait until the Invalidate messages in their queues had been
> fully carried out (i.e., the lines had actually been invalidated).
However, from
http://marc.theaimsgroup.com/?l=linux-kernel&m=113435711112941
Paul E. McKenney wrote:
>
> 2. rmb() guarantees that any changes seen by the interconnect
> preceding the rmb() will be seen by any reads following the
> rmb().
>
> 3. mb() combines the guarantees made by rmb() and wmb().
Confused :(
Oleg.
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