Messages in this thread | | | Date | Wed, 15 Mar 2006 08:48:03 +1100 | From | Paul Mackerras <> | Subject | Re: [PATCH] Document Linux's memory barriers [try #4] |
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David Howells writes:
> + CPU 1 CPU 2 COMMENT > + =============== =============== ======================================= > + a == 0, b == 1 and p == &a, q == &a > + b = 2; > + smp_wmb(); Make sure b is changed before p > + p = &b; q = p; > + d = *q; > + > +then old data values may be used in the address calculation for the second > +value, potentially resulting in q == &b and d == 0 being seen, which is never > +correct. What is required is a data dependency memory barrier:
No, that's not the problem. The problem is that you can get q == &b and d == 1, believe it or not. That is, you can see the new value of the pointer but the old value of the thing pointed to.
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