Messages in this thread | | | Date | Fri, 2 Dec 2022 15:22:11 -0500 | From | Alan Stern <> | Subject | Re: [PATCH] tools: memory-model: Make plain accesses carry dependencies |
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On Fri, Dec 02, 2022 at 05:22:57PM +0000, Jonas Oberhauser wrote: > > but to me OOTA suggests something more: a value arising as if by > > magic rather than as a result of a computation. In your version of > > the litmus test there is WRITE_ONCE(*y, 1), so it's a little > > understandable that you could end up with 1 as the final values of x > > and y. But in my version, no values get computed anywhere, so the > > final value of x and y might just as easily be 1 or 56789 -- it > > literally arises "out of thin air". > > Maybe one can distinguish further between OOTA values (which are > arbitrary, not-computed values) and more generally OOTA behaviors. > > How do you feel about examples like the one below:
There's something wrong with this example.
> void *y[2]; > void *x[2] = { (void*)&y[1], (void*)&y[0] }; > > P0() { > void **t = (void**)(x[0]);
Now t holds a pointer to y[1].
> *t = (void*)(t-1);
And now y[1] holds a pointer to y[0].
> } > P1() { > void **u = (void**)(x[1]);
Now u holds a pointer to y[0].
> *u = (void*)(u+1);
And now y[0] holds a pointer to y[1].
> } > > In this test case the locations x[0] and x[1] exist in the program and > are accessed, but their addresses are never (explicitly) taken or > stored anywhere.
Although they are dereferened.
> Nevertheless t=&x[1] and u=&x[0] could happen in an appropriately weak > memory model (if the data races make you unhappy, you can add relaxed > atomic/marked accesses); but not arbitrary values --- if t is not > &x[1], it must be &y[1].
I don't see how. The comments I added above show what values t and u must hold, regardless of how the program executes. The contents of x[] never get changed, so there's no question about the values of t and u.
> To me, OOTA suggests simply that the computation can not happen > "organically" in a step-by-step way, but can only pop into existence > as a whole, "out of thin air" (unless one allows for very aggressive > speculation and rollback).
All right, this is more a matter of personal taste and interpretation. Is it the computation or the values that pops into existence? You can think of these OOTA computations as arising in a (sort of) ordinary step-by-step way, provided you allow loads to read from stores that haven't happened yet (a very aggressive form of speculation indeed!).
> In this context I always picture the famous Baron Münchhausen, who > pulled himself from mire by his own hair. (Which is an obviously false > story because gentlemen at that time were wearing wigs, and a wig > could not possibly carry his weight...)
There is a comparable American expression, "pull oneself up by one's bootstraps", from which is derived the term "boot" for starting up a computer. :-)
Alan
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