On Mon, Feb 24, 2014 at 11:54:46AM -0800, Linus Torvalds wrote:
> On Mon, Feb 24, 2014 at 10:53 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > Good points. How about the following replacements?
> >
> > 3. Adding or subtracting an integer to/from a chained pointer
> > results in another chained pointer in that same pointer chain.
> > The results of addition and subtraction operations that cancel
> > the chained pointer's value (for example, "p-(long)p" where "p"
> > is a pointer to char) are implementation defined.
> >
> > 4. Bitwise operators ("&", "|", "^", and I suppose also "~")
> > applied to a chained pointer and an integer for the purposes
> > of alignment and pointer translation results in another
> > chained pointer in that same pointer chain. Other uses
> > of bitwise operators on chained pointers (for example,
> > "p|~0") are implementation defined.
>
> Quite frankly, I think all of this language that is about the actual
> operations is irrelevant and wrong.
>
> It's not going to help compiler writers, and it sure isn't going to
> help users that read this.
>
> Why not just talk about "value chains" and that any operations that
> restrict the value range severely end up breaking the chain. There is
> no point in listing the operations individually, because every single
> operation *can* restrict things. Listing individual operations and
> depdendencies is just fundamentally wrong.
>
> For example, let's look at this obvious case:
>
> int q,*p = atomic_read(&pp, consume);
> .. nothing modifies 'p' ..
> q = *p;
>
> and there are literally *zero* operations that modify the value
> change, so obviously the two operations are ordered, right?
>
> Wrong.
>
> What if the "nothing modifies 'p'" part looks like this:
>
> if (p != &myvariable)
> return;
>
> and now any sane compiler will happily optimize "q = *p" into "q =
> myvariable", and we're all done - nothing invalid was ever
Yes, the compiler could do that. But it would still be required to
carry a dependency from the memory_order_consume read to the "*p",
which it could do by compiling "q = *p" rather than "q = myvariable"
on the one hand or by emitting a memory-barrier instruction on the other.
This was the point of #12:
12. A memory_order_consume load carries a dependency to any
dereference operator (unary "*", "[]", and "->") in the
resulting pointer chain.
Thanx, Paul
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