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

[Paul E. McKenney]

On Mon, Feb 24, 2014 at 05:55:50PM +0100, Michael Matz wrote:
> Hi,
> 
> On Mon, 24 Feb 2014, Linus Torvalds wrote:
> 
> > > To me that reads like
> > >
> > >   int i;
> > >   int *q = &i;
> > >   int **p = &q;
> > >
> > >   atomic_XXX (p, CONSUME);
> > >
> > > orders against accesses '*p', '**p', '*q' and 'i'.  Thus it seems they
> > > want to say that it orders against aliased storage - but then go further
> > > and include "indirectly through a chain of pointers"?!  Thus an
> > > atomic read of a int * orders against any 'int' memory operation but
> > > not against 'float' memory operations?
> > 
> > No, it's not about type at all, and the "chain of pointers" can be
> > much more complex than that, since the "int *" can point to within an
> > object that contains other things than just that "int" (the "int" can
> > be part of a structure that then has pointers to other structures
> > etc).
> 
> So, let me try to poke holes into your definition or increase my 
> understanding :) .  You said "chain of pointers"(dereferences I assume), 
> e.g. if p is result of consume load, then access to 
> p->here->there->next->prev->stuff is supposed to be ordered with that load 
> (or only when that last load/store itself is also an atomic load or 
> store?).
> 
> So, what happens if the pointer deref chain is partly hidden in some 
> functions:
> 
> A * adjustptr (B *ptr) { return &ptr->here->there->next; }
> B * p = atomic_XXX (&somewhere, consume);
> adjustptr(p)->prev->stuff = bla;
> 
> As far as I understood you, this whole ptrderef chain business would be 
> only an optimization opportunity, right?  So if the compiler can't be sure 
> how p is actually used (as in my function-using case, assume adjustptr is 
> defined in another unit), then the consume load would simply be 
> transformed into an acquire (or whatever, with some barrier I mean)?  Only 
> _if_ the compiler sees all obvious uses of p (indirectly through pointer 
> derefs) can it, yeah, do what with the consume load?

Good point, I left that out of my list.  Adding it:

13.	By default, pointer chains do not propagate into or out of functions.
	In implementations having attributes, a [[carries_dependency]]
	may be used to mark a function argument or return as passing
	a pointer chain into or out of that function.

	If a function does not contain memory_order_consume loads and
	also does not contain [[carries_dependency]] attributes, then
	that function may be compiled using any desired dependency-breaking
	optimizations.

	The ordering effects are implementation defined when a given
	pointer chain passes into or out of a function through a parameter
	or return not marked with a [[carries_dependency]] attributed.

Note that this last paragraph differs from the current standard, which
would require ordering regardless.

							Thanx, Paul

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