Re: [RFC][PATCH 0/5] arch: atomic rework

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com> 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